Let $G$ be a compact Lie group. I have to prove that for $n$ large I there is an immersion of $G$ in the unitary group $U(n)$. I know that any finitedimensional representation of a compact Lie group is unitary and thus completely reducible. So I have to prove that this representation is faithful... right?
You need to prove that you group $G$ has a faithful representation. This "is" PeterWeyl theorem (or a corollary of, depending on how one states things). 

