First, be sure your Mac is "authorized" for your App Store/iTunes user account. If it is not, and/or not logged in, Share is often disabled.
If that's not it, use "Preference Manager" free from Digital Rebellion (dot com) to trash Motion's pref files quickly and safely.
If that's not it, move Motion from the Applications folder to the Trash. Launch the App Store, Purchased tab, click the "Install" button for Motion.