If you purchased the official DLC on Steam, installation is automatic. However, for those who have downloaded the RUNE standalone repack:

It was a sunny morning in Athens, Greece, and Alexandros "Alex" Georgiou was ready to start his new adventure. A seasoned truck driver with years of experience under his belt, Alex had just purchased a brand-new, top-of-the-line truck, which he planned to use to explore the beautiful Greek countryside and make a name for himself in the transportation business.