Unfortunately it seems I can't. I'm running 10.6, and according to Apple's support article, scrolling only works if you're running 10.5 with Boot Camp 2.2. Snow Leopard comes with BC 3.0 and scrolling is only supported on the late iMacs for 3.0 🙁... Any people who upgraded from 10.5 are SOL.
There is a hack to get it working on Windows using the Magic Mouse 1.0 for Windows update that Apple released, but I haven't tried that nor am I willing to. As I mostly use OS X, I have no need for this and will just wait for Apple to release an update to BC 3.0 for the Magic Mouse. Sorry I couldn't be more help.
Regardless, I'm thinking that only vertical scrolling works. And I highly doubt Apple would implement momentum scrolling for their Windows drivers. If anybody else knows otherwise, please feel free to correct me. This is all speculation as I have no machines running 10.5 to test on.