I open the Web UI of my Computer on my Nexus 5 phone, then copy the key url (e.g. https://link.getsync...=##############). On the phone, I tap on the Add Folder at the top right, tap "Enter a Key" then paste the key url where it says "If you have received a key, paste it here:". Upon doing this, selecting a folder, then tapping "Add folder" an "Unknown Error" toast appears.