TopoJSON requires D3 V3 or higher. If you are on DS 1.4, you'll have to bring in a private version of D3 via some RequireJS trickery, and THEN load your TopoJSON in that context. An example can be found here:
sdkpackage/Choropleth.js at master · org-scn-design-studio-community/sdkpackage · GitHub
Design Studio 1.5 I think has a more recent version of D3 where you might not have to do this, but just make sure D3 is in your <stdInclude> tag in that case.
Let me know if this helps!