In my world the entrance was located in some ruins on the surface near those blocks. If you see a block called a "trophy pedestal" with images of the boss mobs on it you'll know you've found it. From what I understand you need to place one of the boss trophies on it to get inside. I didn't have...
You need to replace the cover beneath the receiver with a panel. The panel will stick the receiver to that frame block. Use covers when you need the frame to not stick to something. You can also use panels to place down redstone wire.
