You may have done this already but here's what I did:
1. got to link and download/save to BB
http://www.mobipocket.com/dev/beta/blackberry.asp
2. unzip
3. USB BB to PC, use APP Loader to ADD mobireader.alx
4. select on BB & grant http connect priviledge
5. do whatever set-up & register required (can't remember this)
6. download/save some free doc (I got demo Paradise Lost) follow instructions
If you have done all this, please list your exact error, make sure you get V. 5.1, I am fairly new to BB as well but this is what I did and it works just fine, perhaps you missed a step or hit a wrong button, did you clear it all out before trying again? Just making suggestions...