Hey man, its a bit different than just doubleclicking like the .xml files, but I'll try to explain
First - Download the skin creator -
http://www.mediafire.com/?8dzfhj7v4ri048g (This version includes the version of Honeycomb Light on my thread as an example)
In the folder you will find the following as shown in the picture below.

The Base xml skin file is the skin on which the custom bitmap controls are placed - this can be your own xml skin file, or one from the forums.
The Compiled xmlc skin file is the compiled file, which merges the base xml file and the custom bitmap controls. (The base xml is NOT needed by musicbee to use the {b]xlmc[/b] skin)
The Images folder contains every image used on the custom controls - The Play/Pause buttons, Lastfm icon, Equaliser icons etc. You can simply edit my example images to your liking so you dont have to worry about the size.
After you're happy with the way your images look, you can view the bitmap skin by starting the SkinCreator.exe. To save the bitmap as an xmlc file, simply press save on the top right corner of the SkinCreator.
Hopefully this will get you started if you have any other questions, just PM right away!
Take care
SOURCE:
Blinghound