Interfacing FPGA with real hardware

Do not worry about the "rden" port. What it does is disable the ROM read logic to save a bit of power. It does not put the ROM's outputs in high impedance, as you need.

In fact, FPGAs only truly support high impedance in the pins' output buffers, so no internal logic blocks actually use tri-state.

So, what you need to do is to add a ROM block, then connect the ROM blocks to tri-state output buffers (alt_outbuf_tri?) and those to the pin.

Add a bit of logic to tri-state the buffers when /OE or /CE is "1".

And you're done.