Just take Math Studies like I'm taking it now. Don't worry, it's not too difficult.
It involves taking an Internal Assessment, meaning a long term coursework. It does not involve Additional Mathematics (with the exception of the very very basics of Calculus, which is only Differentiation). It's basically lots of elementary Mathematics at its easiest. So if you want to do IB, just take Math Studies. However, you should see if the university you plan to get into will accept that subject. Some universities may not, some do depending on what you are doing in the future.