How to choose an Inventory Org from a different OU on Order lines tab?

Hi,

I am trying to implement the drop ship functionality across different operating units in an r12 environment. The user guide says that we can set the security profile needs to be created. I created a global security profile and set it at the user level for the profile option MO:security profile. I still cannot select a different OU inventory org on the drop ship line(with source type external). I have implemented successfully a regular drop ship order with in the same OU,however having some issues with across OU's.

Can some one please tell me what is/are the key difference(s) that one has to do between the drop ship functionality within and across operating units.