I've been given a conditional offer to the program that I want to go in. In order to increase my chances of keeping that offer,if I plan on taking a course in the summer, will they look at that mark and take it in to consideration if my average is lower than what is needed?

Or do they not take it in to consideration because it's too late?

Oh, thanks! Do I have to let them know or will they wait until the end of July anyway?

Basically what happens is when the final marks are submitted, if you don't meet the conditional offer they'll (usually-depending on the school) let you know that your offer is in jeopardy. You need to let them know that you're doing something about it. In this case you would need to go to your summer school and get them to fill out a form saying you're enrolled at the school taking whatever course(s). After the school is finished you need to make sure that they send the marks to the university. Usually you just fill out a form with your university's address and OUAC number. Then (hopefully) you keep your offer!