On your PC, create a jabber account with PSI
Then, connect to your msn account via your new jabber account (there is a "gateway" option somewhere, I don't remember exactly). The thing is that when you create your jabber account, you have to choose a jabber server, and they don't all allow to connect to your msn account. I chose "binaryfreedom.info" and it works.

You should see all your msn contact appear in PSI

Then, just connect to your jabber account on the N900, and it works