OK I can confirm your observations. Your device (and many other WP8 devices) does not have a SD card folder :-(
Stupid enough, WP8 does not allow apps to access the "Windows Phone" folder, so it is no solution to put the file there.
WP8 offers only a very restricted access for apps to the file system.
The only solution for this (as suggested by a Chinese Run.GPS user - thanks) would be to make it possible to download Run.GPS Atlasses through the Gps-Sport Download cart. We'll do work in that direction and see if we can offer this solution soon.
But it's not possible to select the ratlas-Map inside the app. I also tried to place the .ratlas file directly in the root folder ("Phone") and inside the "Documents" folder and even in a sub-directory ("Documents\Run.GPS"). No luck as well.
If you connect the device to your PC using an USB cable you should see two volumes in File Explorer. One of which is usually called "Phone" and the other "SD card" (although the SD card volume is also built-in memory).
Is there any way to use Ratlas maps on Windows Phone 8 ? I'm using a Nokia Lumia 920 and I cannot find any way to copy the .ratlas files to the folder suggested in the user manual.