Mac appstore install button greyed out

Open Launchpad then hold option to make them wiggle. Click the X on the app that is stuck waiting. Re-open the Mac App Store and download again. If that doesn't work, try restarting your Mac.

Good luck with that.

You are absolutely right! CMD+OPTION+ESC will certainly bring up a force quit menu where you can force quit any app you have open. An even easier option is to CRTL+CLICK (or right click if enable) the app that is open for your Dock then hold option to change Quit to Force Quit. Notice the only Apps you can Force Quit are open Apps. This is great if you want to force quit the App Store and relaunch it. But the details I posted earlier are not to quit an open app. Its actually to delete the installation of an app. This is great to remove an app from your Mac or stop a pending (or stuck) installation. Therefore, these two processes address two different things.

Once you find a good stopping place in your work, save all of your documents and files you have open. Restart you mac and try to download it again. If it continues to get stuck, go into the Mac App Store Menu Bar > Store > Sign out. Go to the App and try to download it again. Sign in when prompted. If it continues, try downloading another app from the store (preferably a free one). If it downloads, then you will want to contact either the iTunes Store or the App Developer for this problem App (there is a support link on the right side of the App page on the Mac App Store). If this is happening with all apps, try to download the app using a new user (you can create on in System Preferences > Users). Once the app is installed on the new user, it should be available in your original user. you will still want to troubleshoot your network or current user settings if this works as you will probably run into the issue again in the future. you can use Activity Monitor and Network Diagnostics to troubleshoot those types of issues.

Anti-virus software or LittleSnitch are known to interfere with the Mac App Store.

Others having issues with the iTunes/Mac App Stores have changed to the Google Public DNS service.

Late 2012 Mac mini, OS X Yosemite 10.10.3; Apple Watch, 38 mm silver AL, Watch OS 1.0.1; iPad Air 2 & iPhone 6+, iOS 8.3

make sure your OS is up to date, try logging out and signing back in with your apple ID.