In PSI, under Account Setup, choose Add. Give it a name, like OnSIP. It will then ask for your Jabber ID and your password. Your Jabber ID is your SIP address, e.g. firstname.lastname@example.org and your password is your WWW password.
Lastly, choose the Misc tab to set the chat priority to 5. This will cause chats to only show up in the PSI window and not also in the my.onsip.com window. If you want the chat to show up in both windows, set the priority to "1."