how do I delete an add-on in 2.79? I installed an add-on, it didn't show up so I restarted Blender. It still didn't show up so I thought I'd load it again (I didn't check to see if it was in the add-on list). What happened is shown in the attached image.
Try going to both of the directories and just remove the files completely. Then try installing it again to the Blender Foundation directory, checking every time if the add-on is visible in the list.

Edit: as batFINGER said, deleting the Users...\measureit folder would also suffice presuming that is you main blender directory.