Sounds like you tried to overwrite the the "Base.SC2Data" in the wrong folder to get a better AI opponent.
I had the problem myself, and it's a very easy fix to it.
You got the same "Base.SC2Data"-file in 2 folders just beside each others.
"C:\Program Files\StarCraft II Beta\Mods\LibertyMulti.SC2Mod"
"C:\Program Files\StarCraft II Beta\Mods\Liberty.SC2Mod"
The reason for this, is because the custom AI-file is spesificly programmed for the singleplayer modfolder.
To fix the problem, download backup files >> here <<.
I'm not sure if it's "smart" to delete it since you will need them for the multiplayer emulator later on.