Maintenance for the week of August 3:
• PC/Mac: No maintenance – August 3
• ESO Store and Account System for maintenance – August 4, 5:00AM EDT (9:00 UTC) - 4:00PM EDT (20:00 UTC)
As mentioned here, the DLC is free for all current ESO Plus members or available for purchase with crowns from the in-game Crown Store.
If you are still having trouble accessing Markarth after following the steps in this article then please open a support ticket so that we can look into this for you!