I attached sterling parts by firing them in place. The PMC looks fine but the sterling is dark gray. Did I do something wrong?
Sterling will definitely turn black in the kiln. This is a deeply penetrated silver oxide that must be removed with sandpaper. Tumbling will not remove it--tumbling smoothes (burnishes) the surface but doesn't remove anything. This is one of the reasons I prefer soldering. The black film that is formed during soldering is a factor of time... the shorter heating period of soldering means a thinner film.

What kind of solder should I use?
Silver solder is best because it is strongest, and a torch is required. Bear in mind that you can buy a perfectly good torch about the size of a small flashlight for under $20, so I don't see this as a major hurdle. If you want to avoid silver soldering, the next best thing is a slow melting alloy that uses 96% tin and 4% silver. It is pretty strong, has a bright silver color, and melts around 475° F(250° C). One brand, often sold in hardware stores, is Sta-Brite. Another, sold by jewelry suppliers, is called Tix. Both come with their own flux. I'm confident that with 20 minutes of practice anyone can master the use of these solders. And how do you practice? get a handful of nickels and build a little monument.

What about the solder that comes in a syringe?
There are two "families" of solder, and both come in a paste form mixed with flux. Both will work on PMC but they have significant differences. Try silver brazing alloy is mostly silver and has a melting point above 1000° F (540° C), and is very strong. Low-temp solders are also sold as dust-mixed-with-flux. This will work, but is not recommended for situations where a small point of contact needs to stand up to a lot of stress.