Hello there.
After you've collected the 7 dragon balls and placed them near one another, they will glow yellow, as you have seen.
What you have to do now is stand near them and press B (default wish key). This will bring up the wish window with the 3 usual choices. Immortality, Strength, and Friendship. Just do strength. I don't really care what anyone else says... strength is the best.
A note about the strength wish. If you use strength, MAKE SURE you are not in SSJ or whatever its equivalent for the other characters is. What I mean is, make sure you in the mode you start off at: descended. This is because, if you are in SSJ form and you use strength (which adds 1,000,000 to you PL) you will not gain as much as you would if you are descended because SSJ is your base powerlevel times 1.3 (or some value near that). It's fine if you don't understand it... just take my word for it.
A side note: for all of you who can't seem to get the Capture the Dragon ball mode working on some of the maps, download s2k_greattournament.bsp. It's a very nice map and when I played it, all of the dragon ball locations worked. I was able to make many wishes, ending up with a SSJ PL of 13 mil in a short time. THIS MEANS, to answer your other question, that you can use the dragon balls to raise your PL indefinitely as long as you have a map that doesn't mess up the CTDB mode since the computer bots don't mess with the dragon balls. There is no console command to raise your PL, just use the dragon balls and keep wishing for strength, but remember to be in your base form when you make the actual wish.
Have fun.