Using the 6mm allen key, place the short end of the wrench into the hex key. This plug is sealed in with thread-locker and will be very tight. This is normal. Position the allen key so that it points towards the left, or #9 on a clock. You can use your hand or a rubber mallet for the next part. Hit the allen wrench downwards (towards the floor, or #6 on a clock) with some force. This will break the plug loose so that you can loosen it up easier. You may have to hit it a few times to break free. Once loose, continue to turn the plug counter-clockwise until it comes out of the tank.
Plumbing (Tank Side)
At this point, the hard work is over and the rest is fairly simple. Start by locating the two 1/4" NPT x 1/4" Compression fittings that came with the inflation kit.