For 32(x86)bit systems:

Copy and paste the entire folders here:

C:\Program Files\RocketDock\Skins 

For 64(x64)bit systems:

Copy and paste the entire folders here:

C:\Program Files(x86)\RocketDock\Skins 

Done. Select the skins by clicking on the 'Dock Settings' icon on your Dock.

Enjoy!