Afaik you cannot use Alway One Selected Value on expression. You'll need to create fields for this in the script. Try using the Hierarchy function to create a table with the organisation levels as fields.
I created fields and it still does not work. It treats each selection "Corporate", Group", etc. separately. I want the user to only be able to pick a single value. Not a single value from each drop down selection but a single value for the entire mutlibox.
That's not possible. You'd have to make the user pick a field first and then let him/her select one value in the chosen field. Personally I'd add a Search object with only those organisation fields. That way users can search the value they need in only those fields in that Search object. That won't prevent multiple selections, but if you really want to make their lives that difficult you can add calculation conditions to charts that check the selections or do something with an annoying text box that they'll have to click away and try again to select only one value.