Hamachi, available at https://secure.logmein.com/US/products/hamachi2/download.aspx (Copy and paste URL, pick Unmanaged)
We have a Hamachi network; ask an online member of the group for the ID and password. Note that you're free and welcome to use this network for any games you need Hamachi for, not just Thief 2.
And last, but definitely not least, you need people to play with. That's where this group comes in. Although very few people play Thief 2, much less the multiplayer mod, it's my hope that anyone who's interested will be able to join and use this as a means to find other players.
Also note there's currently a teamspeak server. server2.teamspeak2.pl:8771 pass "garrettloader" If you use that sort of thing.