Aya, HSS supports different Diameter interfaces for different purposes. Looking at your description, you may need Wx (AAA Server - HSS interface). I haven't checked freeDiameter, but you will have to specify this application Id (Wx - its corresponding Id as defined in 29.234 ) while connecting.
In any case, first steps in Diameter connection establishment are TCP (or SCTP) connection and then Capabilities Exchange Request/Answer. You can check at what stage it is failing. If CER/CEA are successful, then you get in to application specific exchange.
Other security related HSS Diameter application (interface) is Zh (bootstrapping). So identify the specific interface you need and look at configuring freeDiameter accordingly.
Alternatives are available. My company has the libraries (Wx, Zh and other interfaces), but it is a commercial product.