Using downloaded skins is easy - here are the steps:
- Log in to your site as admin
- Either use FTP to copy the skin folder you downloaded to the "data/sites/1/skins" folder (where "1" is whatever site number you're on), or click "File Manager" in the admin bar and browse to "/sites/1/skins" and copy the skin folder over to there
- Once your skin is in the skins folder, click "Administration" in the admin bar
- Next click "Site Settings"
- Next find the drop down select labeled "Skin" under the "Skinning & Themeing" section of the general tab on the site settings page
- Select the skin you'd like to lose from that drop down list and it will be applied to your mojo site
Hope this helps!