I have a 955, not a 255, so I can't recreate your exact problem (my 955 supports navigation for trail run), but if navigation doesn't show up the menu for your Trail Run activity (after you hold UP), you could use the following workaround:
1) Choose another sport that does support navigation (like Run)
2) Start navigation
3) Start the timer
4) Hold UP and select Change Sport. Change the sport to Trail Run
It's not a perfect solution since it creates a multisport activity but it could tide you over until Garmin fixes this (assuming it's a bug). You can contact Garmin product support at [email protected] or https://support.garmin.com/ to let them know about this.
My 255 has Trail Run. In case you haven't found it yet, you need to press the top right button and scroll all the way down until it says +Add. Then scroll to running and select it from there.
