This is the tool I got.. Park Tool CT-3. It was about $35, but well worth it. should never have to buy another one again.. ever.

Park Tool Website

Pablo, when you press the chain apart, the pins retain the tight fit they normally have when you press it back in. You just press it back together with the tool. To make sure the chain joint is not too tight, I usually turn it around in the tool and press on the other side just a hair until the chain loosens up and has the same side to side play as the rest of the chain.