Click on Edit in the top menus, then go to Preferences (near the bottom) and open that, then, to the left of the window that comes up, click on Recording and at the top right of that window you’ll see a box to check called Overdub. Make sure that is checked, and that should do it. Then you might have to deal with latency (syncing of the various tracks together). That’s covered in the manual under the Help tab.
in my case - windows xp - it looks slighty different - here’s what you have to do,
in case some user has the same problem.
edit
preferences
audio i/o ( you might have to scroll all the way to the left to find ’ audio i/o’, I had to)
check 2nd last box near bottom saying: ’ play other tracks while recording new one.’