OSM routes don't show up on GPS

Hi All
This is off topic but when I use maps4africa I can plot routes on mapsource and load them into my garmin 2460LT, however when i try and do this using OSM the routes will load on to the micro SD card but will not show on the Garmin (The T4A routes do). Any suggestions please.
I am traveling from New Delhi east through Nepal and Bhutan on a motorbike so I hope I can load these tracks when i get back.
Any advice will be appreciated soon as I leave in 5 days,
Thank you

Moderator note: Split from the original topic.