Make sure when you extract the addon into your addons folder that it isnt nested inside another folder eg. Make sure its Interface/Addons/"Addon name"/bunch of files and not Interface/Addons/"Addon name"/"Addon name"/bunch of files or any other variation.

In this case "Addon Name" would be MultiBoxer and PitBull_MultiBoxer_PartyExperienceBar (theres 2 folders) and not MultiBoxerv2_0.5/MultiBoxer