When composing in Mixcraft6, how do I add Minor chords in my project as needed, for instance in a chord sequence such as : in the key of C, C Am F G the drop down box does not have any Minor chords.
I think you are referring to the drop-down box that's assigned to each clip, and which determines the key signature of it. That's all it does: sets the key signature, not any chords. Whichever chords you require, you create.
Incidentally, if you wish to set the key of a clip to a minor key, just select the related major key.