Hello efekarabent,
First of all, I believe this is in the wrong section, there is a sub-forum for questions/etc.
Also, perhaps there is some confusion here. Themepark.app is an app for changing UI/graphical elements within Mac OS X. If you are looking for help with it, then this probably isn't the best place to ask =/.
If you are wondering about the themes on here, they are for the Chameleon bootloader which allows booting Mac OS X on a "PC" and is not for Macs.
If you are on a Mac, then these themes are not for you, sorry.
If you are running Cham2, then all you have to do to install a theme is:
To Install, copy the theme folder to your /Extra/Themes/ directory, then just add or change the following key-string: In your com.apple.boot.plist (Located in /Extra)
<key>Theme</key>
<string>THEME_NAME</string>