Hi folks,
Nintendo Repair Shop repair tech here. We actually have created a guide to instruct users on how to repair their NES system. The trick with the game cartridge tray is to make sure a plastic latch hooks underneath the NES motherboard. Check out our [http://www.nintendorepairshop.com/nes_repairguide.htm|NES System Repair Guide] for details on how to repair your NES System, and the trick with the game cartridge latch.