This is the second time I heard this in the past few days..
Make a makeshift one! An easy way would be with two people and a rope, or if you are solo you could do it with a strong (very strong) thin rope.
With two people get a rope and put a two or three inch loop on each end at about 39" apart, one guy reefs on the string to bend the limbs while the other slides the string through the loops onto the tips.
The hardest part would be getting the bowstring through the loops without the plastic ends of the stringing aid.
Keep in mind that the string will immediately start to stretch and will require you to repeat this process quite a few times to maintain your brace height.
I don't know why they don't include it either, it's like getting a new car and not getting the keys. The silly little thing is probably worth all of about 10 cents. Hmm, maybe they are trying to lean buyers towards purchasing an extra package..

I would be mad too.
Whatever you do, do not put force on the side of the stock, it will not take it. I can now take off a string without the aid but it involves contorting one of my legs in an unnatural way..