Got another question is there any other way to get the BB messager for 4.5 ?
Posted from my CrackBerry at wapforums.crackberry.com
BB Messenger is a fancy gui for sending PIN messages to other BB users. They are part of the data plan you pay for monthly so no, not part of your SMS/MMS bucket. BB messenger should already be installed in 4.5. I don't know of any US carriers that have restricted BB messenger...yet.
When you hook your BB to your DM, go to update apps (that's what my DM 4.6 says) BB messenger should be there for you to install. If you have your 4.5 OS installed on your computer.
what is a PIN number and how do you add BB users to the BB messenger?
Every BB has a unique PIN, found by going to Options -> Status. To add user to BB Messenger, simply open Messenger, highlight Contacts, hit the menu button/BB button, and choose "Add a Contact".