If the folder is in your Downloads area, you’ve gone too far.
You need to find the link to your purchased download, even if it’s via email or through your account in here. It’s the download link that you right click and save as – not the downloaded folder.
More info at the top here: http://generatepress.com/knowledgebase/install-activate-addons/
This may help as well: http://generatepress.com/forums/topic/just-cant-install-gp-premium-on-my-mac/