If it works similar to how Y! messenger works (or used to) you could like they said attempt to make a direct connection. You could do this by inviting the user to your web cam, inviting the user to photo sharing, sending a file, etc. After doing that you could run a netstat command and it's typical you'd see their IP Address next to the port that the messenger service uses. For Y! My Guess Would Be 5050, so I'd run : (Doing it as if your on a windows box)
netstat -an |find "5050"
In hope of finding my victim IP Address. Good luck though, your best bet would be to do it like how ethicalhacker said it, send them to a site.