Hi Júlia,
Possibly try this.
A) First you will need to turn USB Debugging on the phone.
B) To do this go to Go to Settings > Applications > Development on your phone and click on USB debugging.
C) Now connect to your Mac via the USB cable then drag down the notifications window and click on USB Connection.
D) Now click on Mount.
Hopefully the phone's SD Card should now appear on your desktop and you can drag/drop items to it in the Finder.