Exactly... get a new M1xsp, setup it up as the somfy outlines (speed, parity bits, flow control, etc), then enroll it, set up the multiple text strings (one per command... I have 3 setup per room - up, down, halfway up), then rules which call those text strings).
As I said... you would connect the somfy RTS to your computer to TEST the RTS and it's ability to receive text strings and react to them. You certainly do not NEED to do this unless you have a problem... but, it's MUCH easier to eliminate a big chunk of your hardware and software config before pulling your hair out trying to figure out if it's the automation rule, the text string, the m1xsp, the serial cable, the rts, or the blind motor. Hooking it up to you computer and getting it to react correctly eliminates the serial cable, the rts and the blind motor from the equation... if it works that way, then you know it's the automation rule, the text string or the m1xsp.
I should clarify... this is not specific to the Somfy controller... but, rather, ANY hardware (lighting, HVAC, etc) that you want to integrate and control on any platform (Elk or otherwise). This is (in my opinion) knowledge that you need to have in your toolkit to do home automation as serial control is the BEST way to interface devices, so, knowing how this works and how to troubleshoot it is vital.
So, Hyperterminal is a standard app on windows that allows for you to open a terminal window to interface with a number of different modem-like peripherals through a com port.
A very basic overview is here
http://www.dialogic.com/support/helpweb/divaclient/dc28.aspx
If using vista or windows 7 -
http://digitizor.com/2009/08/29/how-to-ins...windows-7-free/
Not sure how old you are... but, this is the way we used to connect to BBS's through our 300 baud modems back in the 80's
Good luck.