This is SharePoint 2007. I created a custom workflow activity in Visual Studio 2008 (using this and a few other blogs as a reference). I deployed it properly, I think. I can go into SharePoint Designer 2007 and see my action in the list of available actions. But when I click it and press Add, nothing happens.
I've used samples I've found as well as simplifying the code in my activity as much as possible. It doesn't change anything. So I don't think it's my code there, but it's hard to say.