1. Use mixfile remixer to extract GFX.MIX, they are found in C:\Program Files (x86)\Steam\SteamApps\common\MotoGP15 if using Steam, if not it will look something like this C:\Program Files\Milestone\MotoGP15
2. Download the Pack mod files and extract them.
3. Copy and paste the downloaded files (IN THE PICTURES FOLDER IN THE RIDER FOLDER) into the extracted GFX.MIX folders.
5. Repack the extracted folders back into MIX files (make sure to set the format to SBK-X/V8) and save them back into ….\common\MotoGP15 (make sure to back up your MIX files before replacing them).