The value in the installed plugins file is specific to the plugin and basically tells Nop that it is installed. If the value is not there, Nop will run the install code within the plugin. In general, you want this to happen because Nop doesn't really know what needs to be done to install the plugin, the plugin knows this. If the plugin name is in the file, as far as Nop is concerned the plugin is installed and properly configured.
If you want to try the InstalledPlugins.txt file trick one more time, this should be the string you use for the plugin: