You don't need to install the Houston USB drivers for this configuration.

Connect two midi cables from the Houston to your MIDI interface (any port will do), connecting out to in and in to out.

In Nuendo, go to Devices - Device setup - Add/Remove, and select Steinberg Houston and click Add.

Then, on the left side, select Steinberg Houston and select Setup at the right.

This will show MIDI input and MIDI output. Select the MIDI port where the Houston is connected to for both MIDI input and MIDI output. Hit OK and you're done.