I download maps to my Garmin Nuvi from Basecamp all the time.
But I don't use Trip Planner.
See if this works:
Create a list, pick your locations (keep it simple with just 3 or 4 points, add each one to the list.
Your waypoints will be listed in the bottom left window pane.
Select all of your waypoints so that they are highligted.
Right click on them. Then select "Create Route using selected waypoints."
You'll see your route created at the bottom of the list of selected waypoints.
Right click on you're route. Select "open".
A new window opens showing your waypoints. Drag the last stop to the bottom of the list and then select "Optimize"
Exit that window and your route is now ready to download to the Garmin.
Also, you need to check the "options" under the "Edit" menu and make sure that you have appropriate "Activity Profile" selected.
This is a rather brief explanation but I hope it helps.