It can't find your Warcraft 3 installation. This can happen for various reasons: more than one installation, moving it around, etc.
It recommends that you set the path in the preferences pane. I don't remember ever using that, but give it a shot by going to File -> Preferences in the editor and look for an option to set the path to Warcraft 3.
Otherwise, you'll need to run regedit, find the warcraft 3 install path, and edit it to show where your
current War3.exe file is.
http://ccm.net/faq/3814-warcraft-3-registry-error-loading-key-warcraft-iii-installpath