When the whole region is captured why can't the mines be activated and when clicked show red circle
Have you send a unit with right click to capture/build them? (any unit can do that except workers/slaves) slaves you send afterwards with right click, otherwise if you have done this it may be a bug...
Just like Fatarion said, if it has a red circle, it's owned by the enemy. Capture it just like you capture any other building.