Marketpress 3.1 and Support System 2.1.9.1 Conflict

Hello,
I have MP 3.1 installed and I've noticed a weird issue. It actually was happening with MP 3.0.0.8 as well. I've discovered that if you also have WPMU Support System installed (mine is v 2.1.9.1) then the country drop-down box in MP's checkout won't work. You click on it and nothing happens. Any ideas on how to resolve it?
Thanks,
Dave