I went through all of this on my first BB. Took a week before AT&T finally admitted that they did not provision my account correctly for enterprise. If you only have a personal account, you're not going to be able to connect to the BES server (unless AT&T changed something).
I run into this with AT&T all the time, they swear the account is provisioned for BES and it is not. As others have said, your BES admin can try setting your "activation password" on the Server all day long but unless AT&T gets this right it will never work. I just looked it up in our Premier account my work plan is listed as "Blackberry Enterprise unlimted add on" and it is $45/m. Now that AT&T no longer offers unlimted data it may be what pbflash stated, 2GB Enterprise package plus the $15 add on for BES.
