Attributes of Main Entity missing in USMD_SEARCH customizing

Dear gurus,

I'm configuring a customizing for USMD_SEARCH for a Custom Data Model following the help page here:
http://help.sap.com/saphelp_mdg80/helpdata/en/10/0649523e85d130e10000000a44538d/content.htm

When I'm adding the fields to the Selection screen UIBB configuration, I only see the Key fields from my Data Model and I don't have the option of adding the Attribute fields.

For the Resultset UIBB all of them are visible. Have you had experienced this before? Am I missing something that had to be configured beforehand?

Thanks for your help in advance,

Boris