I'm a newbie to Inkscape and I am trying to create SVG files for a cutting machine. When the trace bitmap dialog box comes up, it never shows the preview, and doesn't trace. Any idea why?

Just at the bottom of the preview area is a button or bar that says "Update". You have to click update after each change you make in the dialog, to see the preview of how that change affects your trace. Then when you get it how you want it, click OK. It might take a few to many seconds for the trace to happen on the canvas, depending on the size and complexity of the image being traced. Also, the trace happens right on top of your original image (which I think is both confusing, and unnecessary). So if there is any transparency in your image, you can't see the results of the trace accurately, until you remove the original out from under the trace. It might look like nothing is happening, because they are right on top of each other.

I am using Vista. I just can't get anything to work.

Would you please report the results of trying our suggestions? Once we know exactly what didn't work, and what happened when you tried, we can proceed with further suggestions. Thank you

[Edit -- ~suv, I can confirm that bug on Windows 7. I will add comment to the bug report.]