I was getting the error message even after adding the following links to Available Software Sites:
http://download.oracle.com/otn_software/oepe/mars
http://download.eclipse.org/technology/epp/logging/milestones/
http://download.eclipse.org/graphiti/updates/milestones
https://marketplace.eclipse.org/content/oracle-maf-tools
So I opened the last link, dragged and dropped the Install button into Eclipse window. Some more plugins were prompted for installation, selected all and let them install. Then I had no issues in installing plugins for HANA Tools and HCP Tools.
Thanks Christopher and Ahshan!
Best regards,
Kamal