You need to extract the files if they are compressed into a .zip or .rar, then got to the install directory of Condor (this is often C:\Program Files\Condor), go to the Planes folder, then for each plane which you have a skin for, copy the bitmaps into its respective plane folder in the folder named 'skins'. You can name them what you like but there should always be one with the name of the glider, as this is the default skin.
Handy hint- if there are skins or gliders which you will never use, delete those skins to make the loading of Condor faster. This is especially useful for multiplayer games where Condor has to re-load each time.
Here's a question for the rest of you- can other users see your fancy new sins if they haven't got them themselves? Secondly, I can't use any skins in my version of Condor, I can see and select them in the Flight Planner but then can't see them in the actual game. The wheel etc is also white, and more worryingly I often lack a tailplane