Launch Nuendo
after opening your selected platform of recording....
1) Go to your DEVICES directory and choose REASON
2) at this time, you will see a box of REASON inputs...please enable how many channels that you feel you will use in REASON
***NOTE: the more channels you enable the more processing power it uses***
3) Now launch REASON
4) open the rack modules that you will use for your session
5) enable the module that you would like to use 1st
***NOTE: make sure that your Audio out rack monitor shows REWIRE SLAVE MODE

GO back to Nuendo
1) The 1st MIDI track says MIDI 1 - if you click and hold where underneath the words MIDI 1 there will be a selection of Midi ports. Your REASON ports wilil show up there -

Open your VST mixer -
all of your REWIRE channels should be present at this time...

As you play your Midi controller you should now hear the REASON module through the REWIRE channel that you have assigned...