This is really vague, but someone on the Big Fish forum mentions this:
I have been playing the last two boards on Montezumas level for the past two weeks. So far I have got 4 concrete plates with pictures on them and inserted them into 4 of the 6 holes.
Apparently after you put 6 plates into 6 holes, the game then goes to the endgame sequence and Expert Mode is unlocked.
If this doesn't help, I'll see if I can find more specific information about this.