In first post you have link to complet Garmin folder that you need it.
If apk auto create Garmin folder, then delete this Garmin folder and add inside Garmin folder from first post and you are ready to go when you add also map inside.

For search path of your phone the best way is total commander from google play becouse Astro don't show always correct path.

So, install total commander and when you open it you will see on left side path to all of your SD.
You can take screen shot and upload picture here and we will help you find correct apk for your device externall SD card