I have been trying to install Plus for WLM for a couple weeks now.  I finally got it installed a few minutes ago after uninstalling the Cisco Security Agent (CSA).  CSA is likely to only be installed on corporate computers, and most of those users will not have the ability to remove it... but since that is what seemed to help me, I thought I would post a note about it.

The only other step I took which may have helped was to remove the Messenger directory in 'documents and settings\<username>\Local Settings\Application Data\Microsoft\'.  This directory seemed to have some old data in it from a previous Plus installation, so I thought removing it may help.  I then restarted WLM and Plus still did not load.  I then uninstalled CSA and rebooted, at which time Plus worked.

Hope this helps someone.

-R