To reduce the current from 12amps (1440watts) to 5amps (600watts), you either need to use a transformer to reduce the voltage coming from the wall socket from 120vac to 50vac, or you need to increase the resistance from the design value of approximately 10 Ohms to 24 Ohms.
You do NOT want to shorten any of the heating elements - that would decrease the resistance of the heating elements and increase the current. Just the opposite, you need to find an old heater and add some of the nichrome wire in series with the existing heating element to INCREASE the resistance. That will DECREASE the current, and the total power.
Ohm's Law: Current = Voltage divided by Resistance (I=E/R)
Power = Current times Voltage (P=IE)
Now to the practical issues - if you decrease the current in the heating elements to allow your dimmer to work, your machine will probably not get hot enough to pop corn any more.