Here are your parts
Replacement parts for PLKU1267DS models | AppliancePartsPros.com

The parts breakdown is pretty useless so all I can do is give some general info.

The defrost thermostat should be clipped to the evaporator coils.
The defrost heater is usually mounted under the evaporator coils.
The defrost timer can be in the control module if you have one.
Also they like to put them behind the kick plate or in the machine compartment at the back of the unit. Often all you can see is a bracket with a hole in it to rotate the timer cam and the timer itself is mounted on the back of that bracket.

See the attachment for the wiring diagram.

Are you sure you even have defrost as the wiring diagram also does not show any defrost components?

Is the compressor running?
Usually you can hear it fire up and you should be able to feel it vibrating and it should be warm.

If it running and you only get frost in one area on the evaporator coils (usually where the freon enters the coils) then you are low on freon. This will be an expensive repair as you will need a pro.
They would have to find the leak, repair it and then re-charge the system so there is no guarantee that they will even be able to repair it.
Often it makes more sense to put those dollars towards a new unit.

If it is not running it could be a thermostat problem, a compressor start device problem.

Below is a good link on how to check things.
Appliance Repair Guide: How To Fix a Refrigerator - ACME HOW TO.com