Read the manual

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Ok, seriously now - read the manual. The menus in ED aren't controlled through the mouse. Remember that this is a graphically upscaled adaption of a game from 1984. You move through the menu(s) using keyboard keys (default is, IIRC, WASD) and "press" an option with the spacebar (again - IIRC and as default for keyboard/mouse operation). The default key layouts (or some of them, at least) are shown in the, you guessed it, manual

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And yes, "default". You can remap all of ED's controls to whatever you can connect to a USB port. You can find a few examples of custom mappings at
EDrefcard. That site is also very handy if you just want a list of what your current control setup actually is.