There are two McDonald's restaurants in Mokpo, Jeollanam-do, and are two of the four in the entire province.
One McDonald's is in the Mokpo E-Mart. It is accessible via buses no. 10, 13, 20, 105, 109, 112, 119, and 300. A map of the area is available here, in Korean, from the E-Mart website.
The other location is in Jukdong. A map is available here, in Korean.