Get a desktop Jabber application like PSI
. Install it on your desktop, create a Google Talk account
if you don't have one. Follow instructions or documentation to add your MSN contacts to your Google Talk account. The exact instructions for that can be a bit lenghty, please google for it.
Now log off the desktop application and launch Google Talk in your Blackberry. All your MSN contacts should be there.
There are also Web sites that provide a Web-based gate to MSN. They're just slower since you're reloading a Web page over and over, therefore less convenient than Google Talk.