The current version of TLMP is for testing only, use at your own risk.
You must have the Steam version of Torchlight (v1.15). If you obtained Torchlight through other publishers this will unfortunately not work.
Download the zip from:
Extract the files into your Steam Torchlight folder:
i.e.: C:\Program Files (x86)\Steam\SteamApps\common\torchlight
The Multiplayer will be attached when Torchlight is started and you should see the Multiplayer button in the main menu.
Remove the following files/folders from your Torchlight directory:
Note: You may see a lobby button in TLMP. This is currently not in a working state and you can ignore it.
One person must act as the host for the game. To host across the Internet the host must allow port 5445 UDP to their machine (usually set within the router).
If you want to join a hosted game you must know the IP address of the host (they will need to disclose this to you).
Hosting and joining are similar however you do not need to worry about port forwarding through the router (software firewall issues may need to be resolved on the host). The client can use the LAN IP of the host for connecting (usually in a format of: 192.168.XXX.XXX).
Torchlight Multiplayer Mod - Tutorial