You need to make sure this is set to yes for the marketplace to be enabled on OpenCart.
Token, Encryption string 1 and Encryption string 2
These details should automatically be entered here once signing up. However if they aren't they will be sent to you so you can enter these manually.
Once the details have been entered you will see if the API connection status is succesful. If it isn't you will recieve a red cross with details of why this wasn't successful.