Part of the process for creating a custom action for SharePoint Designer involves create a ".actions" file.

I've found and tried to use a sample template which starts off as:

<?xml version="1.0" encoding="utf-8" ?>
  <WorkflowInfo xmlns="http://schemas.microsoft.com/sharepoint/">
  <Actions Sequential="then" Parallel="and">

However, that xmlns attribute prevents my environment from working.  My actions don't show up in SPD. 

The solution is to strip out the xmlns attribute, do an iisreset and close/re-open SPD.