Simulation is not available when burning to a DVD+r(w). I have a plextor and a BenQ and neither will allow simulation from within nero (BenQ will now do it through Qtools).
I also think that when writing to DVD+ media it cannot be done in sessions as such because the writer does not allow that form of writing. Nero gets around this by putting the drive in DAO mode and writing the session information in one pass rather than several. I could be wrong on this point though.
You should be able to do simulation and Multisession on other types of media.
What type of media are you using?