Don't close the dialog message until you follow the instructions in it. It has instructions on how to get your hires pack installed. It should open up a window with your Aces High folder and will tell you the name of the file to doubleclick to install it. This is a workaround to an issue that is happening with certain Windows 7 setups with multiple users. It's not letting us automatically run the installer. We're working on fixing the underlying issue but haven't been able to duplicate it in house.
Alternately, you can redownload and install the hires texture pack from our website.