First: You need to know your Mac's IP address
Where can I find this?
and whether it's a fixed address or if it gets the address from a DHCP server
How can I find out?

I've got NO idea, since I know nothing about macs. MComb?

Go to your apple menu, and down to Control Panels. Select TCP/IP. Enter these settings.
Connect via: Ethernet
Configure: Manually
IP Address: 10.1.1.2
Subnet: 255.255.255.0
Router: (empty if possible, if not 10.1.1.2)
Name Server: (empty)
Search Domains: (empty)

Like Tony said, be sure to record your current settings first because this is going to disable your dial up connection until you put them back (we can set you up for both later). You do not need to reboot your mac.

Depending on the Mac model you probably don't have a link light. If the link light on the empeg comes on that is good enough.

I do not have a copy of VirtualPC handy, but there is an option for it to share the macs network connection or use its own. There may also be an option to disable networking entirely. These will be under an options or preferences menu. Make sure networking is enabled and tell it to share the Mac's network connection.

-Mike