In the R4 forum here, there is a sticky at the top of the page with the information you need. In short (if you only want to use 1 skin), copy and paste the .bmps and .ini to the _system_ folder
I have a Mac, you can use it with Mac OS X. Just create the folder by taping with your 2 fingers. if it doesn't work click on the Apple logo on the top left, go to system preferences keyboard and mouse, Track pad( if it's a laptop like a macbook or macbook pro) and click on Tap Trackpad for Secondary click.
