After you've extracted the files, copy the folder to:
Kits
Windows: /Documents/Sports Interactive/Football Manager 2015/graphics/kits/
Mac OS X: /Documents/Sports Interactive/Football Manager 2015/graphics/kits/
Tactical Kits
Windows: /Documents/Sports Interactive/Football Manager 2015/graphics/
Mac OS X: /Documents/Sports Interactive/Football Manager 2015/graphics/
Create the folders "graphics" and "kits" if they don't exist.
Then run Football Manager 2015 click on Preferences/Interface
Tick the check box called 'Reload skin when confirming changes in Preferences' and untick the one 'Use caching to decrease page loading times'; then confirm and click on the Reload Skin button.