Before you can do a mashup each post needs to have a MapPress map saved to it. From the code above, it looks like you’re generating the maps on the front end.
If you don’t already have maps saved in each post, take a look at the MapPress geocoding settings. The plugin will let you generate a map automatically in each post based on the lat/lng custom fields (once you have it configured, save or publish the posts to create a map).
Once the maps exist, you can use do_shortcode() to add a map or a mashup shortcode to the page.
There are examples of using do_shortcode() in the documentation. For example, a simple mashup would be: