Step 1
Download the .zip archive and use a tool such as 7-zip for Windows or The Unarchiver for MacOSX to extract the folders.
Step 2
Cut and paste the extracted folder "Andromeda" to:
Win Vista/Win 7/Win 8: C:\Users\<username>\Documents\Sports Interactive\Football Manager 2015\skins
Win 2000/XP: C:\Documents and Settings\<username>\My Documents\Sports Interactive\Football Manager 2015\skins
Win 98/ME: C:\My Documents\Sports Interactive\Football Manager 2015\skins\
Mac OS X: /Users/<username>/Documents/Sports Interactive/Football Manager 2015/skins
Create folder "skins" if it doesn't exist already.
Step 3
Start the game and go to Preferences screen and Interface tab.
You should see "Andromeda" as option in the skin drop down of the Overview box.
Use the Clear Cache button, then hit the Confirm button.