If you're trying to use the Distributed System Designers in the December CTP, I must highly recommend you take a look at the walkthroughs.

They can be found in Help under Visual Studio Team System->Team Architect->Distributed System Designers->Getting Started with Distributed System Designers.

Click here for an image of the walkthrough location

The following is the recreation of the Logical Datacenter used in Walkthrough: Applying Constraints to a Logical Datacenter and Walkthrough: Validating an Application System for Deployment:

Click here to view the walkthrough LDD

Cheers,