Try some of the tools mentioned above to convert to GPX format, then import it to Basecamp and send it, once imported, to the device with this tool.
With my nuvi 510 I have to do this if I want to get these routes avaliable in the GPS:
Go to Tools -> My Data -> Import Route From File
A message is showed with a "Searching for Cust. routes" advice, then a list with all the routes you sent to the device have to be showed; select the desired one (multiple selection is possible) and click on "Import". Once this work is done, your imported route has to be avaliable under "Custom Routes" in "Where to?".
Keep in mind I don't know if both, 510 and 550, have the same interface/options.
Last edited by neke; 31 Jul 2011 at 12:48.