Here is how to install MapMaker levels on your GameCube Memory Card in Dolphin:
1. Open Dolphin.
2. Select "Tools" and then "Memcard Manager (GC)" from the drop-down menu.
3. Click on "Browse" for Memory Card A. 
4. Go to This PC > Documents > Dolphin Emulator > GC, and select "MemoryCardA.USA.raw" in that folder.
5. Click on "<-Import GCI" in the middle section of the menu.
6. Choose the MapMaker level you want added to your memory card.

Place the folders in "GTSE4F" in Documents\Dolphin Emulator\Load\Textures\GTSE4F. This will load all of the mod's custom textures. Also, make sure that custom textures are enabled.
1. Click on "Options" and then "Graphics Settings" in Dolphin.
2. Select the "Advanced" tab.
3. Make sure "Load Custom Textures" is checked.

A guide on installing patches for TimeSplitters 2 can be found here: https://youtu.be/_rmNd-qf9TM