When a new user transmitter is created, it is added to the "FS One" category. This is true for both fixed wing and helis. So, you should see your newly created user transmitter at the end of the FS One category for fixed wing aircraft, if you created a copy of a fixed wing radio.

There currently is a "User Transmitters" category for helis, so when a new user transmitter is created, it is also added to that list if it is a heli radio. However, there currently is no "User Transmitters" category for fixed wing aircraft. We will try to get a fix for this in the next patch (basically, we'll add the "User Transmitters" category for fixed wing).

In the meantime, you should see your new user transmitters at the end of the "FS One" category, for both helis and fixed wing.

One more thing -- if you typically use your own "user transmitter", you can go to the Options Menu, then choose the General selection on the left side. From within the General area, unclick the "Auto-Load Radio/Controller" option. This will prevent an auto-selection of a radio as you select an aircraft. As a result, if you select a user transmitter, it will not be unselected as you select a new aircraft. You may find that this is helpful when using your own transmitter (or a customized radio setup).