How to modify Video position in Sonar Platinum ?

You Cant.

Okay, I just tested this...here's the answer.
 
DirectShow (audini video = 0) - you can move native Windows format videos on the timeline, but not MP4.
Media Foundation (audini video = 1) - you can't move either MP4 or a native Windows format on the timeline.
 
In all cases you can move the start point further into the video. So for example, you can have 00:00:00:00 of the audio start a certain amount of time into the video. This means if you have a blank space at the beginning of the video, you can determine where the music starts within that blank space.

Id save yourself the aggravation and use a video editing software, if you have your basic song structure completed export to mp3 and import into video software to synchronize audio and video, this allows you to easily make changes to video, (or re-export/import audio changes) until you've completed both, then if for some reason? you still want to use Sonar with the video file, it may be easier to align. In any case you'd probably want to use video software to export your final audio/video project to required or various media types. 
Even the basic Windows included/free Movie Maker software may suffice depending how complex your video is.  

Oh I definitely agree Sonar should never try to be a video editor; what I'd love to see though is an improvement in workflow.  Right now I do a variation of what both John and Craig suggest, preparing my video files in Premiere and doing any final striping/output there.  
 
The hassle is, if for example I'm doing sound design or music for a commercial and they recut it (which happens about 4000 times on an average commercial haha), I then need to line it up in Premiere, create some kind of picture in picture to reference in Sonar where the changes are, and make sure it starts at the exact same time (they always seem to put a different amount of black before each version).  If Sonar supported multiple video file layers and the ability to easily line them up, I could have all my versions accessible from within Sonar and easily reference back and forth any changes.
 
I realize these kinds of features aren't really high on the list for most people who use Sonar strictly to create music - but I think it really could become a powerful post audio tool with a few enhancements like this... easier said than done though, I'm sure!
 

dannyjmusic
This is a major pain...I have a video at 00:00:00:00 and have to add a few notes to the front, thanks to the client
so, I need to bump the video forward...when I type in the offset, sonar just hesitates for that amount f time instead of moving the video forward...I'm using an avi file, which sonar seems to like best

 
1. Please read post #4.
2.  You want to slide the video later, not enter an offset. The offset function offsets the start time further into the file, it doesn't offset the file.
 
One more tip: I always convert whatever I'm given to WMV, it takes up a lot less space than AVI (particularly the horror known as uncompressed AVI). It won't affect your ability to create a soundtrack, and you can always export the audio to bring into your video setup for whatever the final format might be.
 
 

What if you create a video file that lasts just a couple of secs with black screen, and import into the project? Put it before the original video file. That should give you some time for the notes you want before the original video starts.. If you can`t import two separate videos in the same project, maybe you can glue them together in "windows movie maker" first..?