If a plugin does feature a wet/dry mix control there are a few things you need to keep in mind. One is that the make up gain control on the compressor only effects the compressed side of that control not the dry signal. So it is important to get the level correct from the dry side first perhaps start with the control set all the way over to dry and get the signal at that point correct level wise.
You now need to set that control perhaps in the middle and adjust the make up gain so you start hearing the compressed side of the sound about the same level as the dry side.
You can have it very geared over towards the wet signal and just add a hint of the dry transients. Or another way to use it is to have it over mainly to the dry side and just add a hint in of the compressed sound. There is no correct way, it just requires you to decide if you want a very compressed sound with a hint of dry or a very dry sound with a hint of the heavy compression. Also it is good to experiment with the compression settings too. Sometime more violent compression works best if you are just adding a little of this to a mainly dry sound. Too relaxed compression may not do much when used in parallel mode.
And sometimes you start out thinking you are going to do parallel compression on say the drum buss for example but in the end you end up just using the compressed side only. Whatever sounds best in context remember.