[spiel2001]

You have to make your addon dependent on nUI (as an optional dependency). That (should) cause WoW to load nUI first if it is present.

EDIT: There is another trick you can do...

if not IsAddOnLoaded( "nUI" ) then
    LoadAddOn( "nUI );
end

if IsAddOnLoaded( "nUI" ) then
    nUIStuff();
end