Several tools are included in Liquid XML Studio for opening external XSD from an open XSD or WSDL file.
When you open an XSD, you should see all of the directly included and imported schema as items at the top of the graphical view. Double click on these to open the external XSD.
When you open an XSD, the Global Component Locator window will show all of the directly and indirectly used external schema along with Elements and Types (you can filter these or just click the first letter to move through the various items). Double clicking an item will display it in the graphical view.
When you open an XSD, if you click "Tools->Show Schema Dependencies" you should see the whole dependency graph. Again double clicking an item will open the XSD.
Option 4 (from a WSDL)
When you open a WSDL, you can click the "View->Show XSD" to display the main XSD, and then use the above methods to see the related XSD.