You need also LabVIEW 6.1 because each version can only go back one step.
In LabVIEW 7.0, save the VI for version 6.1. Now you need to open it in 6.1 to save it as 6.0.
(Saving for the previous version is one of the options under "File...save with options")
If you don't have 6.1, post it here and somebody will do it for you.