Shoutbox

You're looking for .txt (or .xml if they are the default MSNM chat logs).

If it's the Plus! chat logs you could also use a *@*.txt search pattern, which will restrict the search results to only include email addresses. (Or other files with that format, which there may be a few of).

Also, if you installed the undelete utility after deleting the files it may have been written to the same location as the deleted files... meaning they'll be practically unrecoverable.