It’s actually the same template file for both single maps and mashups.
How you do this depends on how the shortcode was programmed, where the shortcode is, and what you’re displaying in the mashup (based on the “mashup body” setting in the MapPress settings or the mashup shortcode).
If you’re displaying the poi body in the mashup, and the shortcode is in the poi body, then the technique above should work.
If you’re displaying post excerpts in the mashup, then MapPress will set up the poi body using WP function get_the_excerpt(). Unfortunately, the WP standard excerpt function strips out all shortcodes. This may be what is happening in your case.
There are various workarounds for that problem. Try a quick search on ‘the_excerpt’ and ‘shortcodes’ in Google or the WordPress codex. Here are few examples that might help: