I was able to load *.mgln files created with "Magellan POI File Editor" version 4.1.1.0 - the one which is on the original CD, coming with the 3225 unit.

Destination of the file:
1 - when you load into the unit internal memory, manually or using the integrated transfer function from the Magellan POI File Editor: "drive":/TFT/USR/CPOI/filename.mgln
It may need a RESET of the unit. However, every time when I unplug the USB cable from my PC, the 3225 unit reset itself.

I noticed that when I tried to use the transfer function of Magellan POI File Editor the default path where the file *.mgln to be saved it was WRONG !!! it gave me: "drive":/TFT/USR/CPOI/filename.mgln/filename.mgln
instead of "drive":/TFT/USR/CPOI/filename.mgln !!!

2 - when you want to use the SD card copy, the *.mgln file in the "root" = No need for any folder !!

I am using and old 32 MBytes SD card (Panasonic).

Note:
- before you do any changes to any internal files, do a FULL back-up of the 3225 unit - copy all files and folders. I see the 3225 as a removable memory of 2Gbytes - aprox. 800 Mbytes free there !!

Entries Limits: 

1. Name must be unique
2. Name and Description retain first 35 Characters only (Not sure about Category)
3. 10 unique categories per file.
4. 1,000 entries per category in a file
5. 10,000 data entries per file