an email to my lab instructor

well, here's the story::
i didnt go the my lab group on tue(28/9), due to medical reasons. and i wish to join the lab group that im not enrolled in. (there r lab groups every mon n tue, but i plan on joining the wk after *11/12 Oct*...reason being...*personal reason* and i juz dont wanna go to my lab. but i guess i cant possibly tell him that coz its my own damn fault for not attending my 28/9 lab.
i figured it would be easier to tell him in person, but he wasn't in his *office* on thur and fri(it seems he was there on both days but i juz missed him).
when i said his "office", it's actually an office for a few(6-7) research students and it says on the door that my lab instructor is an PhD student = =" i reli dont know if he has the authority to decide but...anyway..i'll ask him first.

以上,plz also *hint* that i went to the doctor the next day and i hv medical certificate(29/9)
我send比佢的email要點寫好呢?? 用咩語氣? 洗唔洗formal呢..? 唉..好唔慣...PLZ HELP*
thx a bunch*=)