To enable Marketplace for your system go to the System Settings in the System Admin Panel and check the Checkbox Enable Marketplace 

To select what add-ons are visible in Marketplace check the Require add-on Approval checkbox. Add-ons retrieved from our public repository require approval by the key-user to allow installation in the system.

Go to Add-on repository Source in Marketplace and add as an external add-on repository to retrieve any new public add-ons and add-on updates. 

Go to Marketplace – Manage Add-ons and to Fetch add-ons from the set external add-on repository.

Optionally, you can activate the System Setting to automatically retrieve add-on updates to automate the manual retrieving of add-ons and add-on updates from the public marketplace.