Gee, this is so basic, but people keep asking about it. Maybe I'll make it a sticky. Maybe I'll start recommending that people read the Help.
To the left of each track, there are some buttons. One of them is labelled 'Rec'. Left click 'Rec' and you get a pop-up. You have three options in the pop-up: Left Channel, Right Channel, and Stereo. Don't click Stereo. Click either Left Channel or Right Channel, and you will be recording in mono. (If you click Left Channel, make sure your mic or sound source is hooked up to that input on your soundcard; ditto if you click Right Channel.)