Cell zone for porous medium
 

Hi,
I am a beginner and I am trying to model solar air collector by simulating radiation on a wall (porous medium) and the temperature gain of air through this medium.
The geometry is as simple as it can get:

http://8pic.ir/images/0xyhyspiyea8rgyqmjbo.jpg
First I made a box then used mid plane to create the wall surface and by extruding it at cut material I got my geometry.

meshing is ok as well. (porous medium is meshed)
problem is that in fluent itself there is only two cell zones (inlet and outlet) and i don't see anything for wall, therefor I can not define a porous medium.

I have been trying to fix this problem for a few days now but I have been stuck on this so any kind of help is really appreciated.

Thank you.