1. Go to http://192.168
this is my internal ip, if you don't know yours, start maptool and choose file->connection information, you will see it.
2. Choose Advanced and then Visual Server
my router is Dlink 524, yours might differ but there should be a Visual Server choice somewhere.
3. Click Enable, fill the blanks
the name could be maptool, the private ip is your external ip, the protocol type choose both, the public and private port use 51234.
Again, your router could differ but there should be similar things to fill (I guess, because the player was using different router than mine and I was able to understand him).