It is in your \CumulusMX folder \CumulusMX\Cumulus.ini and can be opened by a good text editor.
It has all the settings that you have set through the Dashboard Settings, and will be updated by CMX as you enter new settings and [Save]. It will also be saved whenever you stop CMX, and that will then have current settings for CumulusMX restart.
Some items in Cumulus.ini can be edited by a text editor like Notepad++ and only with CumulusMX stopped, but generally anything that can be done through the Dashboard Editor should be done there to avoid any chance of it being overwritten at a CMX stop.
Enjoy,
Paul