You need a certificate from sombody like Verisign, or, reading below, there should be a Selfcert.exe program somewhere in the office loadpoint. The selfe signed certs are not a slam dunk, it seems to depend on security settings. I only know this much cause a couple of weeks ago, I was looking for ways to defeat that danged warning myself. Just hadn't gotten around to chasing it down yet.

From Excel's online help (I searched for "signed macro")

Quote:

Digitally sign a file or macro project
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You digitally sign (digital signature: An electronic, encryption-based, secure stamp of authentication on a macro or document. This signature confirms that the macro or document originated from the signer and has not been altered.) a file or a macro project (macro project: A collection of components, including forms, code, and class modules, that make up a macro. Macro projects created in Microsoft Visual Basic for Applications can be included in add-ins and in most Microsoft Office programs.) by using a digital certificate (digital certificate: Attachment for a file, macro project, or e-mail message that vouches for authenticity, provides secure encryption, or supplies a verifiable signature. To digitally sign macro projects, you must install a digital certification.).

If you don't already have a digital certificate, you must obtain one.
How?

You can obtain a digital certificate from a commercial certification authority, such as VeriSign, Inc., or from your internal security administrator or Information Technology (IT) professional. Or, you can create a digital signature yourself using the Selfcert.exe tool.

To learn more about certification authorities that offer services for Microsoft products, see the list of Microsoft Root Certificate Program Members.

Notes

The hyperlink (hyperlink: Colored and underlined text or a graphic that you click to go to a file, a location in a file, a Web page on the World Wide Web, or a Web page on an intranet. Hyperlinks can also go to newsgroups and to Gopher, Telnet, and FTP sites.) in this topic goes to the Web. You can switch back to Help at any time.
Because a digital certificate you create yourself isn't issued by a formal certification authority, macro projects signed (macro project: A collection of components, including forms, code, and class modules, that make up a macro. Macro projects created in Microsoft Visual Basic for Applications can be included in add-ins and in most Microsoft Office programs.) by using such a certificate are referred to as self-signed projects. Depending on how Microsoft Office digital-signature features are being used in your organization, you might be prevented from using such a certificate, and other users might not be able to run self-signed macros for security reasons.

Do one of the following:
Sign a file

On the Tools menu, click Options, and click the Security tab.
Click Digital signatures.
Click Add.
Select the certificate you want to add, and then click OK.
Sign a macro project

Open the file that contains the macro project you want to sign.
On the Tools menu, point to Macro, and then click Visual Basic Editor.
In the Project Explorer, select the project you want to sign.
On the Tools menu, click Digital Signature.
Do one of the following:
If you haven't previously selected a digital certificate or want to use another one, click Choose, select the certificate, and then click OK twice.
To use the current certificate, click OK.
Tips

Sign macros only after your solution has been tested and is ready for distribution, because whenever code in a signed macro project is modified in any way, its digital signature is removed. However, if you have the proper digital certificate on your computer, the macro project will automatically be resigned when saved.

If you want to prevent users of your solution from accidentally modifying your macro project and invalidating your signature, lock the macro project before signing it. Your digital signature says only that you guarantee that the project has not been tampered with since you signed it. It does not prove that you wrote the project. So locking your macro project doesn't prevent another user from replacing the digital signature with another signature. Corporate administrators might re-sign templates (template: A file or files that contain the structure and tools for shaping such elements as the style and page layout of finished files. For example, Word templates can shape a single document, and FrontPage templates can shape an entire Web site.) and add-ins (add-in: A supplemental program that adds custom commands or custom features to Microsoft Office.) so that they can control exactly what users may run on their computers.

If you create an add-in that adds code to a macro project, your code should determine if the project is digitally signed and notify the user of the consequences of modifying a signed project before continuing.

When digitally signing macros, consider obtaining a timestamp so that others can verify your signature even after the certificate used for the signature has expired. See Microsoft Office Online for more information on macro security and timestamps.