Welcome to the club. You need to replace the seat belt for safety reasons. If you remove the safety lock/pin it will never lock again and will be pointless. Got any pics of what you bought? Do you need any other parts? Here is a cheap airbag.
NISSAN PATHFINDER/FRONTIER 04 05 06 07 PASSENGER AIRBAG : eBay Motors (item 190377935418 end time Apr-03-10 12:12:47 PDT)
NISSAN PATHFINDER/FRONTIER 04 05 06 07 PASSENGER AIRBAG : eBay Motors (item 190377935418 end time Apr-03-10 12:12:47 PDT)