You can't select a ASPX Master template. This is the downfall of the ASPX+Portal method, you have no control over the master template through the portal, only the individual pages underneath. Ran into major headaches with this because your master often includes your header/footer, which then means you have to create it with code.
My recommendation is to NOT do aspx+portal. You can implement code behind logic without using ASPX in one of two ways.
- Put your code behind logic in a Control (ascx) and add that to your master page template. Keep in mind you can add values and functionality to the Macro Engine to use this elsewhere on the page then.
- The "Layout" of a page template has the type of ASCX so you can add controls, AND add server side scripts. So you can add a Server side script to your master template through the portal and add any needed logic there.
I would do this before going aspx+portal. But that's me personally, i hate ASPX/ASPX+Portal as requires you to go into code to make changes you shouldn't need to.