SPOILER WARNING !!!!!!!!!!!!!!!!!!!!!!
When you stand in front of the door in the room with the big statue and the waterfall, there is a lever at the right side which opens the door.
This is NOT what I mean. I mean the button on the platform just above this lever. You must climb to where the statue is, climb the next ledge and jump over to this platform.
There, press the button (which lowers a ladder on the other platform), jump back to the ledge, walk over to the other side and jump to the platform with the ladder.
Climb the ladder. You will reach a place with two doors. One door can be opened with a button, don't go there now. Go to the other doow which cannot be opened. There is a grated window through which you can see the golden plant holder and an outside window. You must shoot the window with your revolver, Indy will auto-aim it.
Now go to the other door, open it, get rid of the ice monkeys and find another window which you can destroy with either the revolver or your whip. Get rid of the enemy who shoots from the opposite roof, climb out of the window and shimmy right to the other window which you previously shot.
This way you'll reach the small chamber with the plant holder inside. Hope this helps.