Here is the MTM2 page in Voobly
http://www.voobly.com/games/view/Monster-Truck-Madness-2
Both Voobly and by IP address, you have to Port Forwading(here is another to check your router here https://portforward.com/ports.htm, choose the directx 7, are the same ports)
To host a game:
1. Click Multiplayer, then click Connect.
http://imgur.com/EdK7Q79
2. After that, choose Host.
http://imgur.com/tx1eb0T
3. Name your game, choose TCP/IP Connection(in the image is in spanish) and click ok.
http://imgur.com/fX8Glet
4. Give the IP(you check it here http://www.whatsmyip.org/ it says "your ip address is xxx.xxx.xxx.xx") to your friends, wait your friends to join and you can start playing
http://imgur.com/tsl5J4Q
To join to a game:
1. Click Multiplayer, then click Connect(same as host)
http://imgur.com/EdK7Q79
2. After that choose Join(here it changes).
http://imgur.com/FejfH90
3. Click in TCP/IP Connection.
http://imgur.com/K680jm8
4. Then, type the host's IP(it's in spanish again) and click in Accept
http://imgur.com/DCzVNHA
5. Wait until the game appears, sometimes it is blinking, click on the game, then click OK
http://imgur.com/a/XeEoU
6. Now you can start playing.
http://imgur.com/a/XeEoU
A few tips.
1, have the patch(http://mtm2.com/~mtmg/mtm2.html) and the other players joining or hosting the game to have the patch, if someone doesn't have it, the game will not appears.
2. have a pod program like Cowpod(http://mtm2.com/~mtmg/cowpod/index.shtml) or another one.
3. have the fixmore4.pod in the pod list.(http://mtm2.com/~mtmg/utilities.shtml)
Happy gaming!!!!!!
Extremo.
(Thanks SLO_COPE to helping me doing the joining part)