It sounds like Lashey naturally carbonated the beer in the keg via priming sugar and it came out a little over carbed. By loosening the lid (or pulling the pressure relief valve if you have one) you released that extra pressure, and possibly (depending on how much you loosened the lid) introduced some oxegyn into the keg. The lower head pressure in the keg then caused some of the co2 that was in solution in the liquid beer to come out and fill the air space.
For serving from a keg, you are always going to need some sort of external force to move the beer from the keg to the faucet. Typically this is done by introducing co2 (as you mentioned) to push the beer out. You can also use a 'beer engine' or similar to serve using a 'cask' or 'real ale' strategy (all the stuff 'in single quotes' might be handy to search on if you want more info) in order to serve, but these methods will also introduce o2.
If you just try to serve beer out of a naturally carbonated keg with no way to re-introduce pressure, you're won't get more than a few glasses before it runs out of pressure.
I'd sugest picking up a co2 tank and a cheaper regulator. Once it arrives, you'll want to read up on serving pressure, line balancing, etc.
Edit, you may also want to checkout these sticky posts in the kegging section. There is a lot of good info on how to get started with kegging. It's a bit of a learning curve (it took me a few batches to get my pressures right, etc) but totally worth it.
Best of luck!