Input assignment is handled on the Mix Preview page as you begin to run mixes, the second page listing your carrier and product(s) and their intended amounts. Select the product you need to assign an input to -if one is already assigned it will show that as you view the list, no need to select the product to confirm that it's assigned. 

You may leave the product UNASSIGNED if you DO NOT want the Mixmate to run that product and you are manually adding that product to the mix.