1. If you had ever installed it before, you will need to uninstall it and reinstall it. (You would know this was the case if it did not ask which version of SketchUp to use)
2. Go to the SketchUp menu: Windows / Preferences / Extensions and see if RpWallMaker appears and make sure it is checked.
3. If it still does not appear in the Pugins menu:
3a - open the ruby console
3b. type is: load 'rpwallmaker_loader.rb'
and hit enter to see what happens