So, I presume the NA map is in a folder called map which resides in your download folder on our computer.....
If so, using Explorer or some other file manager, remove the NA map folder from your sd card.
Then, with the sd card installed in your unit and the unit on and docked to the cpu, run TTHome.
In the menu option of TTH, go Device-->Change Installed Device and select the sd card.
Now, in TTHome select 'Add Maps/Traffic/Voices'. you should see listed the NA map on your hard drive. TTHome should show its location as 'computer'. Select it and choose install.