This plugin wants to use /tmp directory of your hosting server as a cache directory. For most hosting setups this will fail.
For example, my Mediatemple hosting has seriously screwed up permissions because the apache user is not the owner of the directories and cannot delete the cache files.
Therefore, until the cache directory can be placed elsewhere this plugin is for certain server configurations. If you don't know what this means, then don't use the plugin.
And don't use MediaTemple because of the permissions problems.