App Store says app is "INSTALLING" but nothing happens
Maybe this thread on Apple Support Communities is what you're looking for.

It sounds like it is, indeed, a cache problem. From the thread above, it'd seem you'd need to find the cache in the /private directory. I'll include specific instructions, inspired by the thread:


Open Terminal (/Applications/Utilities/Terminal.app)
Type in this command: sudo find /private/var -iname "com.apple.appstore" and press Enter
For each of of the found directories run rm -r &lt;dir path&gt;/* (For the brave: sudo find /private/var -iname "com.apple.appstore" | xargs sudo rm -r)