The ICAO code is entered on the "Equipment" tab of the Aircraft Information window. To get to this window, click on "Aircraft Info" in the Tools menu, select the aircraft from your stored aircraft profiles and click the "Edit" button at the bottom. With the Aircraft Information screen open, select the "Equipment" tab at the top.
On this screen you can enter the FAA and ICAO equipment codes. You can also use the check boxes and drop-down lists to let the system "build" the code for you based on what equipment you have in your aircraft. As you select options, you will see the code being generated for you in the ICAO Equipment Code entry box near the top of the window.
Here is a screenshot of the Equipment window:
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