I haven't bought the kit yet,but it's a matter of weeks before I do

I hope to use it for the Playstation 3 game Rockband 3 as Pro Drums, which accepts MIDI drums.
I'm just not sure how to get the MIDI feed from the computer to the MIDI input on the Rockband gadget?
I assume I need to get a sound card with MIDI out, but is it as easy as that?
Thanks!