I bought an aircraft with the 10A and tried to update the firmware to 4.0 but I can't find the serial port on the unit. I have the serial to usb cable.
The serial port is not on the unit, it is part of the cable that the installer builds or purchases. Theres a chance the builder didn't install one. You'll need to talk to them to find out where it is, or if there even is one.
We sell it as part of the the complete EFIS harness for $90.
If your EFIS is already installed, you'll probably want to download a copy of our installation guide from http://www.dynonavionics.com/docs/support_documentation.html and build in the connection yourself.