In order to play custom maps you will need to follow these instructions.
1. Open red orchestra and go to the steam workshop in the main menu
2. Browse steam workshop and subscribe to the appropriate map, eg. Ogledow.
3. Go back to the steam workshop and click on the play map button in the bottom right hand corner. You will then be taken to the red orchestra main menu and it should say 'downloading Ogledow' in the top right hand corner. Once the download is complete, you will be able to enter the game in the usual way.
HAPPY GAMING!