What id you
download Plus! 5 again and simply install it a second time? Run the installer twice in a row if you have to. I know it sounds strange, but a few people reported that worked for them.
If that doesn't help... does
Plus! 4.9 work on that system? That version may not show the Plus! icon because it isn't fully compatible with WLM 2011, but it should at least display 'Plus!' in the menubar.