Need to add new workflow actions to SharePoint Designer? About to edit C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\1033\Workflow\WSS.ACTIONS?
As WSS.Actions is a core file, it should not be modified.
Instead, create a new file with a .actions extension in the same directory (e.g. MyActions.actions), and using the same schema as WSS.Actions add a definition for your custom activity.
<?xml version="1.0" encoding="utf-8"?>
<WorkflowInfo Language="en-us">
<Actions>
<Action Name="MyCustomAction">
... definition...
</Action>
</Actions>
</WorkflowInfo>
After an iisreset your .actions files will be read and provided your definition is correct, the activity will be available in SharePoint Designer.