A couple of pointers.
Make sure your custom project post build event is copying your .aspx file somewhere below the mojoportal web project since it has to run in the context of that project. ie the file does have to exist on disk at the url you are using. Look at post build event for other projects to see how to do that
the stuff you are trying to use was really meant for popup dialog pages. Typically you need to pass the pageid and moduleid in the url to the dialog page in order for it to have the same page context as the cms page from which it is loaded
if you are not trying to make a dialog page but just a normal supporting page for your feature then you should not use the dialog class but instead use mojoBasePage and the normal master page. ie don't use a dialog page from the blog as your guide, use one of the other pages like ViewCategory.aspx. Again for this kind of page you also need to pass the pageid and mid (moduleid) in the url to the page to keep the context of the cms page that contains the entry point usercontrol of your feature.
Hope that helps,