Download latest version of WP Pro Membership add-on from your Dashboard → Purchases Tab. Install the downloaded file through plugin installation system. This is an add-on for Download Manager, so Download Manager plugin has to be installed and active to use WP Pro Membership.
For any technical issue, if you are already using pro version please post in pro forum and free version users please post in free forum. Otherwise, if you have any pre-sale or order related query please contact live chat support team. For technical support.