Hey Jen, if you want to take a course in another uni you need to fill out a letter of permission (lop) form and have it signed by your department/faculty. After that, you need to take the letter to the university that you wanna do summer school at.
I am not entirely sure if there are other steps in between. Talk with your academic advisor, i am sure they will know exactly what you have to do and will tell you all about the deadlines and such.
One more thing, you need to get at least a 4 (C-) in the course that you take at the other institution for it to count towards your degree at mac. Just something to keep in mind.