Before you make changes to your board it is always a good idea to back up your database first.
Extract the zip files to a folder on your computer.
Upload the image folder "skilltest" to your forums images directory (ex: forum/images/skilltest).
Login to your Admincp in your vBulletin Forum and Click on "Style & Templates" to expand the menu.
In the expanded menu click on "Download / Upload Styles" , a new page will load.
In the new page look for: "Import Style XML File" click on browse and point it to the .xml file that you want to install
If you are using a vbulletin version older than 3.7.0 select "Ignore Style Version"
Press IMPORT to complete the installation