Mixers are found by the asset and scanned into the system for ease of referencing mainly. To use one just use the Mixer.? struct helper or pass in its name as a parameter into a mixer edit module. You can set the default mixers for both audio & music in the asset settings provider in the project settings which will apply to all clips unless specified.

// Standard play from the library with a mixer from the library in use. 
AudioManager.Play(Clip.MyClip, new MixerEdit("MyMixer"));