I am having trouble dragging and dropping python scripts from a custom 10.3 toolbox into a modelbuilder model in ArcGIS Pro 1.0. Is there a way to activate this capability?
This works for me. Are you trying to drag it from the Project Pane (either Folder or toolbox node) into Modelbuilder?
Either make a connection to the folder the TBX lives in, or from the Toolbox node, right click and add toolbox pointing to your existing tool.
If you're trying to drag from ArcMap to Pro, this wont work.