RIS 2.0 uses the USB IR tower, and although several people (including myself) have tried getting it working under Virtual PC, I have not heard of any success yet.
If you use a serial based tower (along with an appropriate USB/serial converter if your Mac only has USB), then you can get RIS 2.0 to work. However, you will need to run an Applescript to configure the serial port emulation in VPC to mimic the Mindstorms handshaking. You will need VPC 4.0 or above (5.0 for OS X) in order for the Applescript to work.
Due to some complications involving the Mac's serial port, the RCX Code software does not always run correctly under certain emulators. For example, Virtual PC 2.x was incompatible with the Mindstorms software. This didn't stop Dave Hylands or Scott Taylor - they have both found ways of patching a portion of the Mindstorms software so that it will run under Virtual PC 2.x. Additional information on the patches can be found below.
Personally, I prefer Scott Taylor's patch since it only modifies a single file (SPIRIT.OCX). This is probably the easiest way for people to run the RCX Code software under Virtual PC.
In the meantime, Connectix has improved their serial port emulation, and users of Virtual PC 3.0 should be able to run the Mindstorms RIS without patching it. However, the COM1 ports must first be configured for special handshaking, and since there is no UI to do this, you need to run an Applescript to configure the port each time you start VPC. I believe Connectix includes a script on the CD-ROM. I wrote my own which you can download here.
NOTE: When I first upgraded to VPC 4.0, RIS stopped being able to communicate with the RCX. Connectix suggested that I upgrade to 4.0.1 and re-install the "additions" from within Windows. They also suggested turning off Virtual Memory. After upgrading and reinstalling the additions, communication with the RCX started working again. (I was already running with Virtual Memory off, so I'm not sure if that matters)