|You cannot. Not without hacking into Yahoo's server I certainly would not recommend that. |
Once a user set themselves as invisible his status is controlled by the Yahoo chat server. If there were a way to see invisible users it would be pointless now wouldn't it.
When you are talking to someone on a chat server you do not have a direct connection with their computer. Both of you are connected to the chat server and the server acts as a relay passing on messages between each user or not if they are marked invisible. It is for this reason that you cannot discover an users IP address. A remote users IP address gets no further than the chat server.