You should add the Computer Science subject into your study plan under "Breadth No Maths" - any breadth you decide to do that does not have an MAST prefix should be added here.
No idea if it still applies, but when I was in first year they only let you put second year subjects into this slot, though they've probably fixed it since then. It's worth mentioning that sometimes what you can do on the study plan is a little different to what's actually required by the course. If you have any problems with fitting the subjects in where they should be able to go you can always chuck the student centre an email who can brute-force it in there for you (provided it's allowed within the degree rules).