When you have attempted to uninstall certain plugins in the extension manager, you somehow get this error:

Plugin Uninstall: Manifest File invalid or not found

Solution:

First of all make sure that you have done all of the normal "uninstall" things for your plugin, then

***back-up your database first***

01. Access your database
02. Find the table called "jos_plugins"
03. Find the "record" for the plugin you're having problems with and delete it

* Please take note that you would just need to remove the one "record" instead of the "table"