To use bidirectional ports in EDK, custom IP cores need the signal_I (input), signal_O (output), and signal_T (tri-state) ports in the HDL and MPD files. XPS then automatically connects these individual signals to an IOBUF at the top level.
The Platform Specification Format Reference Manual (psf_rm.pdf) has a detailed description of this situation.This document is located at:
<IDS_INSTALL_DIR>\EDK\doc\usenglish\psf_rm.pdf
See Chapter 3 MPD -> Design Considerations -> Tri-state signals.