I have a 550 and took out the sd card because I couldn't figure out how to download routes and maps, feel free to PM me, Garmin wasn't much help, I was able to upload pictures, which seemed like a waste of time.
OK -- For all -- I just tested my first custom route and it works fine
Basically you need to do the following:
1) Have the Nuvi 550
2) Update the maps (if it is new, you get to update your maps for free up to 90 days after your first start on the GPS). When updating the maps, there will be an option (in the .EXE program) to download to your computer AND gps. I recommend doing both so that you can plan your route on your computer without the GPS being plugged in.
3) Open your GPS and push in your micro SD card
. It takes up to 16gb (got mine at staples for $20 on sale)
4) Download Garmin BASECAMP
-- this is the new program that is taking the place of "Mapsource" if you see that in your own research
5) Open basecamp. If you downloaded the maps to your computer, in addition to the GPS, you should see "City Navigator North America..".
6) If you have question on how to use basecamp (it isn;'t the easiest) there are online videos by garmin for tutorials.
7) Import a GPX or create your own route in Basecamp. Again.. I could help you if needed ,but overall try to watch the garmin tutorials
8) Once you create your route.. (still in basecamp program), plug your Garmin GPS into your Computer via a Mini USB to USB 2.0 plug
9) Basecamp should autoreconize the nuvi 550. If it doesnt you need the new drivers which can be found on garmin;s website.
10) Once basecamp see yours nuvi, it will show on the left hand navigation bar.
11) Find the route you created -- most likely in "My Collection" folder on the left navigation bar -- > Right click it --> send to --> Nuvi 550.
12) Once its done uploading, feel free to disconnect!
13) Start your GPS
14) Now you have to load the route from the SD card to the internal memory. How? On your garmin, go to tools - > My Data -> Import Route from File -> Select 1 or all of the routes you transffered.
15) Wait till it is done uploading (takes a few seconds)
16) Ok now it is loaded to the memory of the device
17) At the garmin main screen -> Where To? - > Custom ROutes -> Pick one that you imported
18) Next option will be "Would you like to navigate to the start of the custom route" -- If you pick NO, then it wont do anything until you get to the start. If you Pick YES, it will navigate you to the start, then once you get there, it will load the Custom Route. This is for instances where you did not plan your route from your home location, but lets say at the start of a state park.
If you need, I can create a really quick video tutorial of the whole thing.