I opened that PDF file linked to in your last post, and the lock nut (part 6B) appears to have 6 sides in the drawing, with two notches on opposite sides of the lock nut.
If that locknut is hexagonal, then it is meant to be turned with a basin wrench as suggested in previous posts.
However, if that drawing is misleading, then you can find a tool that will tighten this locknut at any hotel in your area. "Faucet wrenches", or more specifically "Beer tap coupling nut wrenches" are used to tighten the beer tap to the draught dispensing system in bars. Every barkeeper will have one. Take a look at the tool with the red plastic handle on this web page:
The tool is curved to fit around the lock nut. At the end of that curved section is a single "tooth" that engages a hole in the coupling nut that holds the beer tap in place.
Alternatively, just ask the owner of any bar or hotel in your area who repairs beer dispensing systems in your area, pop into that place and the guy there will probably just give you an old faucet wrench free, or sell you one for $4 (full retail price). If it's got "Budweiser" or "Coors" printed on it, he probably got it free.
I make my own beer, and I have a magic fridge that dispenses cool home made beer 24/7. That's how I know about beer taps.