So you've checked to make sure you've wired the pins correctly and are using the RS232 input on the RSTII NOT the RS485 input?
The com port has the right setting for the Somfy?
There is no carriage return (\r) after the code?
You can operate the blinds directly from the buttons on the RSTII?
I've used this piece before and it's worked just fine. I just entered my own codes for all the channels.