For MP3gain see http://mp3gain.sourceforge.net/
There are two related issues and MP3gain and the PlayIt live processing do different things.
MP3gain will (as far as is possible) make the subjective loudness of all your MP3 files similar. Other than an overall gain shift the file is unaltered.
The PlayIt live processing attempts to make the files sound "better". There are plenty of parameters to change and I recommend caution as it is easier to make things sound worse than better with processing !
I would start off with a bit of gentle AGC and see how that sounds on decent speakers.
PlayIt Live seems to be "neutral" to sound as far as I can tell. The sound is faithful to the source files.
If you want processing there is a plug in or you could use external processing.
If your problem is level differences then try MP3gain.
If you want "creative" DJ software look at Mixxx. That has much more adjustment but isn't really suiited to radio playout.