Adding Users to Local Security Group from other Domain

I have 2 domains internal.
DomainA is a "regional" domain with trusts to External partner site Domains C,D and trust to 2nd Internal DomainB.
DomainB is Primary Domain with Users, Pc's etc.

I want to add users to an AD Local Security Group from Domains C,D on DomainB but I can only see Locations as far out as DomainA (where the trusts are setup).

If I add a Local Security Group in AD on DomainA, I can see Locations from DomainsB, C, D.

any suggestions on how I can "see" Locations on DomainB for Domains C,D??

Thanks

In your ISA Server and VPN can be more helpful. I had provided an link below which will take on the detail page of its settings and configuration. Along with the setup stages you will also get some troubleshooting help in the end to deal with most common errors. Click on the below link and go through the page.
How to Join or Access an Internal Domain from an External Client Using ISA Server and VPN

Carley, I may have a similar issue in a thread I posted about four days ago. Could you kindly please look into it?