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.

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