Logitech k750 with OSX ML -- FN key twice does not access "dictation menu"

[Daffy]

Hi,

does anyone here have a Logitech K750 keyboard with Imac and OSX 10.8.2 and the same mistake like me ?
FN key works but, when I use it two times it does not give me the option to dictate.

if I map another key it works. Logitech could not help me so I decided to ask her if someone has the same problem.
Sorry for my bad english; I'am French.

Yann