How do I link a Post Type to a Template in Masterplan?

I have never used Masterplan nor Masterpress, however as far as the template hierarchy is concerned, it seems to be no different from the standard WordPress way of dealing with it – See Masterpress Docs

I assume that if your (Custom) Post Type is correctly registered through Masterpress interface, it should be registered in WordPress.

So you probably just have to send your PHP template, renamed to single-{post_type}.php on your sever, in your current theme folder. For instance if your post type is registered under “trees” (take not-rewrited slug, or front-end name as it seems to be called in Masterpress), then your PHP file should be named single-trees.php

It should then be used automatically by WP when accessing a post from this Post Type.