Hi,
The installation instructions:
The level replaces Coastal Ruins. 
There is no custom level script. You can play it immediately when playing, by selecting the Coastal Ruins in the default level editor installation settings, when rewriting the "coastal.tr4" with mine, or rebuilding the .tom file, or outputting the wad from the project file into the a brand new custom level. :D

Enjoy.

Jeffrey van Oort.