What defines a post as portfolio page content?

This theme probably uses the custom post type feature. Therefore you have to create a post inside this custom post type menu. The picture below shows you the label “Cars” as a custom post type. You probably have a label “Portfolio” or something similiar here.

custom post type

Another option would be, that you can set the type of a post inside the post editor screen. Create a new post and look for a metabox where you can specify the type of this post.

If this doesn’t help, you should contact the developer of your theme.