alright Deano?
this could be what Todd and Drew say - "first play assignment"... but then your simulator should tell you that when you click it(!?!?)
I've attached a screen grab of a DVDSP file - in the graphical tab on the left side are my 4 tracks and 1 menu...
the top left icon has a little small disc and arrow icon in it's top left corner - this denotes that it's set up to be the first play track. By right clicking on any of the tracks (or menus) you can set it up to be the designated part of the disc to autorun from... (you can also setup the firstplay in the inspector and elsewhere.... you may already be happy with using a different method...?)

IMPORTANTLY THOUGH:
to check whether this is a software / programming / hardware thing - as Drew says, you can check the BUILT files on your computer before even attempting to burn.... You will need to find the two AUDIO and VIDEO_TS folders being built and saved to your drive by DVDSP.... by default they will have been thrown into your Docs
You can then open DVD Player and File->Open DVD Media.... browse to your VIDEO_TS folder and select it... nothing will happen (usually) so then just click the play button on the DVD controller....
If your software is working okay and if your DVDSP project has been programmed correctly then the "DVD" will play just fine.... At which point we'll recommend a few other options - Roxio Toast for eg? / Reinstalling DVDSP? / checking whether your drive is burning anything else without probs?
I've never had probs with DVDSP - always been one of those happy seamless relationships - I'm sure you'll get to the bottom of it shortly... frustrating but you're nearly there!! 
Good luck and remember! "It's never a problem - only a challenge
:)