Sometimes while installing and uninstalling an add-in, it may be the case that the following error is received:

"The feature you are trying to use is on a network resource that is unavailable.  Click OK to try again, or enter an alternate path to a folder containing the installation package 'Sperry Software - (name of add-in).msi' in the box below.

The error is shown in the screenshot below, in this case for the 'Timed Email Organizer' add-in:

This error is the result of either a previous uninstall that ended up incomplete, or possibly an install that was done on one Windows profile and the uninstall attempted on another.

Regardless of how it happened, Microsoft makes a tool called "MicrosoftProgram_Install_and_Uninstall.meta.diagcab" that can correct the problem.  Simply download and run this tool, and select the add-in that is causing the problem.  

After running the tool, you should be back to square one!