we have updated the documentation with a tutorial on hierarchical designs. This should give the basic information required to achieve a fully working hierarchical circuit.
The hierarchical design is intended to simplify and reduce the complexity of your final architecture re-using existing components.
As happens with every component, the user has to insert the input and output pins also in the top-level circuit, otherwise the VHDL generation cannot be completed.
Let me know if documentation can solve your issues.