Simply install into your Blitzkrieg directory. No need to select Run/Data nor Mods folder. It will install a couple of .pak files in the Blitzkrieg/Run/Data folder which can easily be recognized by their name. Concerning the patch, I haven't found any other method so far as to open the .pak files and replace the files manually with those from the .zip.

Uninstall through the Control Panel.

While Achtung Panzer! is installed you will not be able to play the original German campaign. This can be played after Achtung Panzer! is uninstalled or removed from the Blitzkrieg folder. No file will be overwritten during the installation, so after you uninstall or directly remove Achtung Panzer! from the Blitzkrieg folder, the previous state will be restored.