How do I add MyType as a default page for a folder?
A MyType content object exists in the folder, but it doesn't show up when I use the "select content as default view" action.
You have to add MyType to the list of default page types (default_page_types lines property) in the ZMI at:
http://your.plone.site/portal_properties/site_properties/manage_propertiesForm
Add the friendly name for MyType to the list. You have to check the "Title" property for the MyType in the portal_types tool (in the ZMI) to get its friendly name. Now you should be able to add your custom type as the default content for a folder using the display menu.
See also the FAQ for setting the default index document. and "How do I add to the list of default display choices for a folder?"