You should be able to do it through your scroll wheel.

im having the same problem! :(

You have to be standing in front of the civilian, rather close. Then you will see a prompt in the middle of the screen, like, "TALK TO MAN." When you see that, press spacebar, There won't be any conversation, but time will pass and you will get credit for intelligence gathered. Better get out of the area fast after that, because there might be an enemy ambush. If you're taking helicopter transport, put the LZ a good distance from the interrogation site.