There is an Allen wrench inside the cajon. How do I use that correctly?

Use the Allen key for the Allen bolts which are located at the bottom of the cajon (same locations as the rubber feet. They are located on the playing surface side. Clockwise will tension the strings tighter—counter-clockwise will loosen the strings. Adjust to your liking but do not make them too tight. If you notice a lot of tension increase, stop.