well its simple. just follow this steps!
1st - go to ur facebook account and activate the mobile account, then facebook will be sending u an @chat.facebook.com account. this is the account u will be using to log in chat from ur n900
2nd - go to jabber.org and register an account then link ur @chat.facebook.com account via jabber
3rd - go to ur n900 voip account then put ur @chat.facebook.com account in the jabber.
4th - congratulations u have facebook chat on ur n900

nid more help? email me ill chat help u frederick_lopez2000@ovi.com