Custom Edit for Sharepoint document library

I have a document library with 2 different Content Types. I need to create custom edit form for editing document properties, I need different custom edit forms for content types.. So I followed the instructions SPD support site to create the edit form using Sharepoint designer. I have done this of several lists and it all works perfectly.

For the document library when I try to attach the custom form to t开发者_JAVA百科he library I don't see the "Content type specific forms" drop down to choose the content type to attach the custom edit form.

How can this be done?

I'm using MOSS 2007

If using 3rd party components is an option, you can use PowerForms to easily customize the edit forms for each content type without using SP designer