Amy,
I also could not get discount codes to stick on the Hertz website. I typed the Promo code (I did not try Gail's suggestion - the Club Code).
After several tries finally I gave up, and called them direct. I called both Hertz normal number and the special Hetz/AAA number and got the same rate, which by the way was a few dollars less than the online rate. I got a one week rate (Mar 30 - Apr 6) for $179 (midsize) - all taxes included, I believe the rate for a compact was about $7 or $8 less.
The morale of my story - you don't always get the best rate online.
Jeff