Make sure you are launching MCC with anti-cheat disabled. When you go into either Custom Multiplayer or Creative (theater or forge) in Halo Reach, you will get the option of Official Maps or Modded Maps. Slipspace includes two modded maps called Zentih and Eclipse.

A video guide for installing and launching the mod would be nice.

Launch MCC without anti-cheat, go open Halo Reach Forge, and the maps should be there. If you want to join a game, join the discord server, and there are farther directions and people to help you out.