How to: Add Activities to the Toolbox (Legacy)
When building a workflow solution with the legacy Windows Workflow Designer that targets the .NET Framework version 3.5 or the .NET Framework 3.0, custom activities can be added to the workflow project and their designers placed in the Toolbox for easy access. You can also add activities directly to the Toolbox from a dynamic-link library (DLL).
To add an activity to the Toolbox from a DLL
Right-click the Toolbox window surface under Windows Workflow, and then click Choose Items.
In the Choose Toolbox Items dialog box, click the System.Activities Components tab, and then click Browse from the lower right-hand side of the window.
Select the DLL from the file directory that contains the implementation of the custom activity to add to the Toolbox, and then click Open.
Click OK to finish adding the activity to the Toolbox.