The highres is a self extracting EXE file. It should automatically list the AH install directory when it asks you to unzip. Most times you don't have to change that. I just hit "next" and it's done.
Then make sure you have set it to 1024 (double check settings) and it should work.
Here's an easy way to tell if you're at 1024 or 512: Launch a B-17 offline and look at the cockpit. Compare to this:
To aid in comparison, note your in-game cockpit placards, which are all part of the bitmaps. Note the logo on your steering yoke. They become much clearer on 1024 resolution.
