I do not have the link in Plugins menu.
In Manage Plugins dialog the libenvvars plugin is enabled.
I'm not sure whether I get you right, but you have enabled the envvar plugin and cannot access it through the plugins menu, right?
If that's true, then try Settings -> Environment -> scroll down to "envvars plugin" (or similar). Not all plugins are accessible through the plugins menu. This menu is usually for plugins that trigger some specific actions but not for setting things up (as the envvar plugin).
With regards, Morten.
