Make sure
You are plugging your Garmin into the same usb port as when you installed the software. My computer does not detect my device otherwise. Its kind of a process to get anything into a GPS. You have to just mess around with it and try lots of things until you learn how to do it. The MB agent is for uploading files to motionbased not downloading them to your GPS. Just download the .CRS file from motionbased and then import it into your Garmin Training Center, and then to your device.
I really hope future versions of Training Center will have the ability to convert files between all the formats, and make the downloading and following of courses more user friendly.