The pivot rod seems too short to catch the end of the drain plug. I found some instructions and it says to loosen the pivot nut. I did so but the pivot rod will not move. There seems to be a pivot rod seat in front of the pivot nut. I am afraid to do anything with that for fear of causing a leaking pipe. Please help me. I am a widow and need to do a lot of things myself. Any help is appreciated.

Loosen and remove nut around rod. Pull rod back. Drop stopper in drain with flange toward the rear (toward the rod). Reinstall rod and tighten nut.

Watch this video, ignore up to 3:17 when he shows the position of stopper.

How to Replace a Sink Popup Drain by Dave Klec - YouTube