Interior lighting - problem with sun's photons

Hi,

I am trying to do an architectural interior with mental ray and want to have a
few area lights on the inside and a rather strong sun coming in through the
windows. 

However, I can't get the sun's photons to function. mental ray says that no
photons from the sun is stored. I have used the photon radius option and limited
it to just include the windows but it didn't seem to help.

Should I use a sky dome?

Any links to tutorials would be great.

Best,