Sorry, that’s not something supported by the plugin. URLs often do look funny, and passing the whole string is the way Google normally does it.
I have two ideas if you really want to change it:
1. If you set the address to “lat,lng” that may do what you want in the URL – but then you’ll also see “lat,lng” in the directions form.
2. Another option might be to just put your own directions link into the POI body (assuming you’re opening a new window at Google). Just disable the normal directions (there’s a setting and shortcode parameter for that). Then enter the link with the lat/lng instead of the address. You can get an original link to modify by just copying one that MapPress generated.