sending a message to a specific ip address

If you know the person, ask them to look in system preference/network, what is their computer name. you will need to go into system preferences/ Help and click index and read up on , and search for an answer there. and also look into Sherlock/Apple care, there you will have access to Apple vast library on just about any subject [Apple] related. its been awhile since i traveled that path.
You wil also need to devide the focus and choose one medium, either ichat or file share, using text edit as a message medium or some script for mail.

You can do it via various means, but not without some action on the part of the target - the ability to blindly send messages to some unsuspecting user is too easy to exploit (and has been done many times in the Windows world via 'net send'...).

If you want to use something like iChat then you'll need an iChat server, and you send messages by username, not by the IP address of the machine they're on.

Alternatives include AppleScript (which you can use to 'display dialog' on a remote system that you have privileges on), or even something like Growl, both of which need actions by the user to enable.