their account creation process is not the best.
- either go directly to thei website: http://www.soundamus.net/
- or click the Create Account button in MB in the Internet & Services preferences page which will take you to the same page
- it asks you to enter some artists first, by clicking a link 'enter your favourite artists' or click the last.fm icon if you use last.fm
- after entering the artists it will ask you to create an account
using the account id you created, enter that username into the id text box in the MusicBee preferences page
if you cant figure it out its probably best you approach the soundamus fourm which has a link on the bottom of their page