How to get the "Encrypt external SD card" option to show (on a LG Optimus F6)?
I don't know about those exact models, but on Jelly Bean in general, you must enable a password on your phone to encrypt external SD.

Security  -> Set up screen lock -> Password.

Then you'll need to set a password for the SD.

Step-by-step tutorial with images:
http://visihow.com/Encrypt_Files_on_Your_SD_Card_Using_Your_Android_Phone