iMovie '09 iSight not working

I'm sorry if I don't describe this correctly -- I'm the opposite of tech-savvy!

I'm trying to record a video using iMovie, but I get an error message each time: "No camera connected. To import video, please connect a camera." (I'm using iMovie '09 version 8.0.6 on an early 2009 iMac running OS X Yosemite, version 10.10.3) But my built-in iSight works when used in every other application, including when I try to record straight from YouTube. I know my iMac and my version of iMovie are both horrendously old, but I'd thought that since I'm running the latest OS, I'd be able to download the latest version of iMovie from the App Store for free -- not so!

I've been looking around the Apple Support pages for a good hour trying to see if anyone else had this problem, but iMovie '09 seems like it's not frequently discussed anywhere. I tried clicking links to an Apple support page in multiple Support Community answers describing my exact problem...and the links invariably went straight to a 404 error page.

If there's a way I can get around this problem without paying for a new version of iMovie, that'd be wonderful -- but any advice anyone can give is more than appreciated. Thank you in advance!