jgmoose,
You can right click on the plugin from the My Software list under the Software tab and remove it.
How are you trying to install the plugin? Is it through an update site?
When there is no internet you cannot use the update sites. In that case you can download the plugin and add it using the ‘Add from Archive File’ if it is a zip file or use the ‘Add from Local Folder’ if it is an extracted folder.
Hope this helps.