If you mean the bottom of the tower near the beginning of level 3, then you don't have to move any blocks. Just walk into the room, turn right and climb onto the block, turn left and jump straight up. Indy should grab the edge of the air duct. Someone else had this problem awhile back so maybe it is a bug that is fixed by the patch. Get the patch at www.lucasarts.com
and be sure to follow the instructions given there for using it.