I was going to link to the relevant part of the VirtualBox documentation, but it turns out that the documentation is terrible.

So:

- In the VirtualBox Manager window, with the VM stopped, go to Machine / Settings / Storage.

- Under "Storage Tree", next to the listed controller, click the little button that looks like a CD with a "+" on it. It's at the top. Ignore the buttons at the bottom.

- In the dialog that appears, click "Leave Empty". This will add an Empty CD/DVD drive to the VM. Click on it.

- In the "Attributes" pane on the right-hand side, click the little CD icon. It lists "Choose a virtual CD/DVD disk file" and "Host drive blah blah blah". Choose the "Host drive".

- Then, go to the "System" panel in the same dialog. On the "Motherboard" tab, there's a "Boot Order" list. Ensure that "CD/DVD" is checked, and that it's at the top.

Note that this is from VirtualBox 4.3.18, running on my Linux box, but it should be more-or-less identical on the Windows version.