I have a project with a few workflows. I created a custom activity and I've added it to the same project.
But then I found that I am not able to insert/drag it to my workflow.
The only solution, that I found, was to create a separate Activity Library, move my custom activity to the library, and add a reference to the library from Workflow Project.
It looks like that you unable to Refer to Activity from workflow in the same project.