After trying to sign up for Houdini Cloud Compute (following the Hqueue Guide), and not seeing anywhere to get signed up for Houdini usage, Support helped identify a gotcha:

Turns out that after clicking the Sign Up button (under Render > Cloud Properties), and getting signed up with AWS, you need to go back to the Cloud Properties window a second time and click the Sign Up button. This will then take you to the Houdini Cloud Compute page, where you can get signed up and continue following the Guide.

SESI are working to add this to the AWS Market Place, to make it easier in the future; but in the meantime, this worked for me, hopefully it helps anyone else hitting this too!

Thanks Support!