How to install the MLPPack:
Place the folder called addons into the garrysmod/garrysmod directory.
Example: C:\Program Files\Steam\steamapps\jummkopf\garrysmod\garrysmod
Yours won't be jummkopf; it'll the name of your account.
When you're in the directory, just place the file. It'll transfer. Then, say "Yes to All". It'll just place the MLP files into the game. Then just play the game and spawn it. You may need to go into the Browse tab if it doesn't have its own spawn category(I haven't played Gmod yet)