open up mystic realm in WXE and extract AGZ3 to a new wad. then rename to map to an interger between 01 and 99 and have a look-see in a map editor (wadauthor or doombuilder).
And if you havn't been to agz3, let alone downloaded MR yet, then I'll try and explain here...
1) Make a triangle. (or a square, doesn't really matter.)
2) select one of the sides and tag it X (with X being any new number)
3) set that tag to 'Trigger Linedef executer (continuous)'
4) select another side of the shape and tag it X aswell.
5) set that tag to 'intangable, translucent' and set that side's main texture to 'THZPIPE'.
6) select the 3rd side of the shape and tag it Y (Y being a NEW number)
7) set the tag to 'Teleport player to tagged sector'
8) Meanwhile, somewhere inside the stage, make a new sector and tag it to X. This will act as the Teleporter. Set its sector tag to 'Linedef executor (no floor touch)'
9) Far away, where ever you want the teleport to lead, make a new secter and tag it Y.
10) Finnally, add the thing 'Teleport destination' within step 9's sector. If you followed everything correctly, you should see a portal similar to the ones in MR. when entered, you should teleport away to wherever you put step 9's sector, at the specific location of the teleport destination, faceing the way it was.
Happy?