Symptoms: You transfer a custom route to your Garmin nuvi but it doesn't appear in the Custom Routes list.
Cause: Garmin forgot to mention that you need to "import" the transferred file before you can use it.
Solution:
Get the route on your PC. Either download a GPX file or create your own custom route using a tool like Garmin's BaseCamp (replaced MapSource in 2012). Free download here: http://www.garmin.com/en-US/shop/downloads/basecamp
Using BaseCamp, transfer the custom route to your GPS. You can save it to the devices internal memory or to a SD memory card in the device.
On the GPS device, click Tools > My Data > Import Route From File. Select the route(s) you transfered in Step 2 and click Import. Your route(s) is/are now ready to use via the Where To? > Custom Routes feature.
Enjoy!!