You should be able to capture and drive a Dimension that is controlled by a Global Variable using DriveWorks. In the image you have shown, it looks like the Master Model is open, not the newly created model. The dimension will only be driven to the new value in the newly created model.
Because Global Variables are a very basic form of automation, and there functionality can be mimicked in DriveWorksXpress, it is recommended that you remove all equations and global variables from a model, and move the logic over to Xpress. This will make your automation project more manageable as all your rules etc will be in one place.
There is now a DriveWorksXpress Training and Certification that you might want to check out to learn more.
I hope this helps.