You can define the order in the MDX statement

{National,Potential.[Very High]:Potential.[No Potential]}

But I assume that you want the members to be in the correct order when
you drag the dimension onto a pivot table control of some sort. I have a
couple of suggestions, you could try.

Both involve creating a real "national" member in the dimension
(obviously no data will link to this one directly) You can then setup
the sort order in the dimension table as you would normally

Then try setting up either a calculated cell or a custom rollup formula
on your "national" member. If it were me, I would probably explore the
calculated cell option first.

HTH

--
Regards
Darren Gosbell [MCSD]
<dgosbell_at_yahoo_dot_com>
Blog: http://www.geekswithblogs.net/darrengosbell