We had our TV mounted and installed a month ago and the installer said I should get an in wall extension cord because I couldn't run the tv's power cord through the wall. But when I googled the in wall extension cords it said that they weren't permanent fixes. What do I do to conceal the TV power cord?

There is a product called wire mold... it attaches to the wall, the wire fits in it, and there is a cover to conceal the wire. It can be painted to match the room color... that's one option.

Install the receptacle behind the TV.
Arlington Industries makes boxes specifically for this application.