Services are useful! Here are the steps again.
In Automator choose Service as your document type and save it with the name you want (something like 'Uncheck Checkboxes').

Drag a 'Run AppleScript' action into the right pane:

Copy and paste the script into the action, replacing all the default text there, so after hitting the "hammer" compile button it looks like this:

Change 'Service receives' to 'no input' and 'in' to 'Numbers.app' (you have to choose 'Other...' and navigate to the app in the main Applications folder, not the iWork '09 subfolder) so you get something that looks like this:

Save and close. In Automator File > Open Recent and choose the service you just saved. You should see this:

Click 'Install' and if necessary 'Open Anyway' in System Preferences > Security & Privacy, giving you this:

The service should now appear in your Numbers > Services menu. Done.
Then, if you do this so often it would be useful to assign a keyboard shortcut, go here in System Preferences > Keyboard:

Click 'add shortcut' and type the shortcut you want (that does not conflict with existing shortcuts).
These steps are easier than this description makes them appear. If you have trouble, post and I'll set up the service so you can just download the .workflow package and double-click to install.
SG