While they are not quite VU meters, the meters don't require any add on and from my experience "just work".
I take it the songs play OK.
My best guess is that, for some reason, the files are low level.
Is there anything "unusual" about the song files ?
If you use a commercial MP3 file do you see the meters indicating ?
The meters should just work. You need very quiet audio before you don't get any flashing on the meters.
Very curious, I take it you can hear the mixed output of PlayIt Live on the computer (all the players are set to a computer sound output) and it all works OK.