#1 Always be polite. You know what being a cashier means, customers always complaining, don't take these guys down even more.
#2 Have a friend to record a video while you order
#3 When paying, simply ask if you can "pay with McDonaldsCoin cryptocurrency" . Regardless of answer you get tell the staff it's a new form of payment not implemented in their store yet, pay as usual and go. Avoid kiosks of course. They will ask colleagues and their manager and it will spread internally. Remember, always be short and polite, maybe give them a slip with website address if they ask what's that coin.
#4 Upload your video and post link here and in other channels made in near future for this purpose.
#5 Try only once per store/staff member so they won't be annoyed of you, repeat in a week if ran out of stores in your area.
#6 Be patient, it's not 4chan and reddit, but people, this may take weeks to break out to media