You can use berry vine, which does connect you to MSN instant messenger, for a 7 day trial period before you need to purchase it. BlackBerry Meesenger is free but that's for pin to pin messages. I don't know of any MSN IM apps for the BlackBerry that allows you to use for FREE. I only know of FREE AIM messenger which is via Ramble if you use AIM. Otherwise, everything that I know that you can access MSN messenger has a fee associated with it.
I have my AIM, Yahoo and MSN contacts on Google Talk and it works fine. I don't have a clue what the process was to be able to do this but it was extensive and did include the help of a third party software other than the chat clients. Do a search for google talk, there was a how to post in one of the threads.
I've tried every conceivable way to have MSN on my Blackberry. Even some non-blackberry specific Java apps. They all worked, but I was not happy with many things.
I even tried the Googletalk/Jabber way. I tried having friends use Yahoo (cause the Yahoo messenger is free) but I didn't like the way it vibrated after every single message (even when in the window)
I've decided to bite the bullet and pay the 35$ to Verichat which appears to be the best suited for me. I love how it pops up automatically on a message and I like the features. It stays connected 24.7 for days and days at a time.
The two downfalls are BES requirement (for Bell) which is ok now as I have a free work BES but in the future may not be so good.... and that it's 35$...
I hope by next year a more suitable app will exist.