You'll need a Connection & Credential Alias as well as the credential itself. From there you should be able to populate the credential alias in the connection details on the REST Step in IntegrationHub. Once you've done that you can access the credential itself from the credential value data pill. 

We have a really good overall IntegrationHub training course on the developer site that walks you through setting up C&C and using it in IHub and we also did something similar on a Live Coding Happy Hour episode a few weeks ago around the 51:30 minute mark.