Welcome Clovis, to the best darn DIY'r site on the web.
I believe the "P" trap they are referring to is external to the expansion coil at the air distribution (furnace) device. The coil should be piped to a nearby drain, and trapped to seal off any air leakage.
When its all said and done there is usually more said than done
Just use the bleach in cooling season Clovis and a cap full is plenty. Bleach sitting in the trap without getting flushed by condensate fairly quickly is hard on coils. In cooling season the trap will be flushed with fresh condensate water after a few minutes of run time.
Typically, it's the 3/4" PVC pipe, it comes out of your coils that are attached to your furnace. This line drains the condensation from the coils. The P Trap is a U shape in the line. In the line you may see a piece of PVC sticking up w/wo a cap. This is where you pour your bleach.