I ran into quite the mystery while building custom workflow activities for SharePoint 2010. It seemed that, no matter what I tried, the new activities would not appear in SharePoint Designer.

At length, I realized that the assembly name did not get resolved in the .actions file. Visual studio left the token $SharePoint.Project.AssemblyFullName$ as-is.

I found a solution on MSDN, which indicated the Visual Studio tools only replace SharePoint tokens on a few file extensions. To register additional file extensions, such as .actions, edit the project file and add the following at the end of the first <PropertyGroup> element.

<TokenReplacementFileExtensions>myextension;yourextension</TokenReplacementFileExtensions>

That did it for me. From that point forward, my workflow activity appeared in SharePoint Designer 2010 as expected.

Cheers!