To be fair, once you set it up, it’s very easy to buy stuff from them (BFD expansions are the same thing, and that’s where my experience comes from.)
The trick is inMusic’s Software Center. You need to have an inMusic account, which is connected to the Software Center. Once you redeem the freebie from your account (mail), it shows up in the Software Center for download, installation etc.
It’s not that different from other companies, except that they don’t make it clear to the user that inMusic Software Center is the “hub” that controls all software brands under their umbrella. (AIR, AKAI, BFD, Alesis, Sonivox, M-Audio, etc etc)