Makerbot´s Replicator Desktop has it´s filament spool holder enclosed within the machine which is nice. Due to this only a special spool will fit. And so, if you want to use another spool your options are somewhat limited.
This Spool holder mounts on the back of the existing spool holder once it´s in it´s loading position.
Pull the filament tube so that the end of the tube is on the back of the machine enabling the filament to enter the tube from the back.