a couple of points to get you on the way.
Firstly make sure you are using Samplitude V10.1, the latest updates make a large difference.
Assuming you have already rewired Reason and successfully have a MIDI signal or track feeding Reason, you will need to create a separate audio track in Samplitude to recieve and record the audio.
Set the 'audio in' box on this track to whichever outputs from Reason you need. ( click on the 'audio in' box on the Sampy track to select Reasons outputs from the drop down menu).
You should now have audio on this track, if you cannot see the meters moving try toggling the speaker icon next to the track record button.
If all is good to this point then I would suggest having a look at Kraznets excellent tutorial on 'MIDI to audio'.
This will give you a good idea of some of the several ways of getting audio from a MIDI track. I use 'freeze track' often and reliably (remember to freeze the Sampy audio track not the track with MIDI data).
Actually recording audio from Reason in real time just doesn't work on my system (Sampy Pro v10.1 on Intel Core2 Duo E6600, 2.4Ghz with 2 x 1024MB of RAM) although it should!
I would love to be able to as the wave form is more managable than in a freeze track.
If anyone else can help with an insight to this I would be grateful. I have audio from Reason feeding an audio track in Sampy but can never record more than a blank object - 'freeze track' works every time on the same track.
Sorry for the piggy back post Sam, maybe someone can help us both out.