I don't think you can use Global Variables with Mindstorms NXT. But if you are not using a NXT kit for your project, then, first, post in a different forum , second, I think there is a vi for doing that. I'm not sure at all, but I think I've seen where they initialize variables with a special vi. I have no other idea of what it could be, except that I very most likely saw this example in one of the LabVIEW Basic CDs that came with LabVIEW (mine, at least).
Hope that helped.