The URL for the Neo4j web UI is listed in your Graphs List. Each instance associated with your account will have an entry that looks like this:
The URL entry shows a clickable link to your instance's Neo4j web UI. You can also access the Neo4j web UI from the Actions menu:
Either way, your Neo4j web UI should look like this when it first loads up:
You can then enter the username and password for your Neo4j instance to connect. Don't use the defaults – they won't work!