I'm having the same exact problem.
The only part I'm wondering is if I downloaded the correct Sphax version.
I downloaded the 1.5.2 version since that is the version I see when I launch Hexxit.
I then extracted it into a folder, then copied over the path (both the original and patch were the 128 version).
When I view it in the Texture pack section, it's greyed out, I can click on it, and after a minute or two it then gives me the option to click on Done. Yet no changes in the textures.