Mapster is the level editor. You will need eDuke32 to play this, a modern source port of Duke Nukem 3D which permits maps with up to 4 times the sector and wall limit. You can get it here.

It needs the file "duke3d.grp" from the Duke Nukem 3D game folder, copied into the eDuke32 folder to work. Specifically Duke Nukem 3D 1.4 "atomic edition" because the map uses many textures from that version which aren't in the earlier 1.3d version of the game.

Then you can optionally install the Duke Nukem 3D High Resolution Pack to modernize and beautify both the graphics and music.

After this you launch eDuke32, go to new game, then to "user map" and select this map from the list that appears.