I send the same question to Atmel and they responded the followings:

"The first time you start our device it automatically goes into
in-system programming mode.
You simply have to connect RxD and TxD to the corresponding lines of
your PC RS232 port with the suitable line drivers.
To reprogram the device, you shall pull PSEN low with a drive
equivalent to 200 ohms and reset the device (p2.6, p2.7, P3.6 and P3.7
shall not be driven low during reset)."

And it works!
Thanks again for your suggestions.
Gilles Dehin.