JMP stores dates as the number of seconds since January 1, 1904. As such, you must supply a day, you just don't need to display it or use it.
I noticed that you had your month and year columns listed as character strings. They probably should be numeric, but this formula will work with them as character string:
Notice that the Date MDY function requires the numeric Month, numeric Day, and numeric Year values to be entered. I used the first day of each month as the default. Then use the column info on this new column to set the format to be numeric, and the format of m/y. I have attached the new table with the formula in it.
Dan Obermiller