[installation drive]:\[MBAC installation path]\replay
Melty normally names the folders replayXX, where XX is a number from 00 to 99. However, you can use whatever name you want for them, since Melty just needs the files inside and doesn't give a shit about the folder name. Personally, I rename them to YYYYMMDDHHmmss, to know exactly when it was I had that specific match.