iMovie does not support DVD-corders.
so, Plan A) swap it to/buy a miniDV camcorder, that's what iApps are made for..
Plan B) ... the painstaking route...:
* make sure, the disk is "finalized" in that device (read the manual of recorder ...)
* if it is NOT standard sized, do NOT insert such a disk into a slot-in drive... that could get seriously damaged... miniDVD only in slide-drives... if your Mac has a slot-in drive, buy an external dvd-drive...
* finally, it needs a conversion as any DVD, you need 3rd party ($$) apps to do so, your options in recommended order:
*
DVDxDV (free trial, 25$, Pro: 90$)
*
Apple mpeg2 plugin (29$) +
Streamclip (free)
*
Cinematize >60$
*
Mpeg2Works >25$ + Apple plug-in
*
Toast 6/7 allows converting to dv/insert dvd, hit apple-k
* connect a miniDV Camcorder with analogue input to a DVD-player and transfer disk to tape/use as converter
non of these apps override copyprotection mechanisms as on commercial dvds...
http://danslagle.com/mac/iMovie/tips_tricks/6010.shtml
http://danslagle.com/mac/iMovie/tips_tricks/6018.shtml