Big mixer is a bit outdated. I remember people talking about bugs.
You should update your scope or use another mixer.
To make midi CC, you right-click on a parameters, move a fader of your hardware controller, and click the " + " sign when the right CC number appears in the Midi CC display.
If this does not work, then the device is not midi CC compatible for a reason or another.
When done, you save a preset in the "MIDI" bank of the preset list, and then save the preset list to disk... but it is more secure to also save a project (because presets can bring a discrepancy between the actual value of the parameter, and the apparent value of that parameter (example: a fader will have an actual value of -in dB, but the fader will show another value... some devices will however show the actual value of the parameter in any case. This happens on faders, as i have noticed myself, on some devices, not all, because it depends on midi implementation). So to avoid this kind of stuff, it is better to save a project (+ it makes your default project sexier if it is like "launch scope and automate immediately").
other tips:
Additionally, BEFORE doing any midi CC assignment, you should save a preset with zero CC assignment, because it allows to switch midi mapping easily between devices (ie, "reset" or "re-map" at will).
When you save a preset, it is a good idea to save different presets (like, save the first 32, then another preset with 64 CC, then another with more etc...).
Because you can't have more than 119/120 CC by default (if you exclude CC"0, CC64, and the ones above 119, but this is more relevant for synths (64 is sustain), and by saving your work progressively, you have a chance to "undo" the last steps if you find something that does not work (or if, later, you find yourself wanting to change the assignments to other parts of the same device, but need to come back to your original mapping, or need to change some parts only, so also think about what you are going to assign first). you can't assign every parameter due to the limit imposed by 120/127 max CC per device.). Excluding values that are used inside synths to perform features like sustain and so on, seems not relevant for a mixer, but it actually allows building a single midi template (in your hardware) that you will be able to map to any kind of device, be it, mixer, fx, synth....
Here, I just have a single page of a Zero SL "user template" assigned to the most used features of my mixers (that's 54 parameters in total, but i use 32 only, and if I need more, i can add another page in the template, but I never need more, in fact, I don't need more than 24 to have that i need to play, mix and master). I use this template for everything including, devices, cubase, vstis, dj apps, whatever i need to remote control, that's just one single midi template for everything.
Happy mapping
