Andy,
Edit > Set Tabbing Order sets the tab order for the highest-level controls on the panel (i.e. your clusters). Tab order within a cluster is controlled by the cluster order (right click on the border of the cluster and select Reorder Controls in Cluster).
In run mode, to select elements within a cluster when the cluster has the focus, use CTRL-Down Arrow. Then you can tab within the cluster. Use CTRL-Up Arrow to give the cluster the focus again, and tab to move between clusters.
The menu names above are for LabVIEW 7.1 and may differ slightly for 6.1, but the principle should be the same.
Steven H.