There is currently no built-in way to display custom fields (although it's quite a good idea!).
You will need to add them to the mobile theme. I just refactored these a little, so before you start, I would recommend you get v1.1.9 or later.
The plugin installs its own themes, so once installed, go and edit the files in /themes/mobile_pack_base/... The main function for displaying a post is wpmp_theme_post inside index.php. You could put your fields somewhere in or around:
print '<p class="entry">';
I hope that makes sense... good luck.
Oh, and when you upgrade the plugin in the future, you'll want to make sure you either a) don't upgrade the themes to overwrite your changes, or b) reapply your changes to the new version of the themes each time.