Is the scheme $(r, Mac_k(r) \oplus m)$ CPA-secure?
Your first approach was almost correct. Your question says nothing about the shape of the secure MAC except the length. The secure MAC only says that you cannot forge a new tag from the previous ones. 

Let $MAC_1$ be a secure map with output length $n-1$ than

$$MAC_2(m) = MAC_1(m)\|0$$
will be a secure MAC with output length $n$. 

Now, put $MAC_2$ into your scheme and use 0 to distinguish the messages. The adversary sends two messages $m_1$ and $m_2$ such that $\operatorname{lsb}(m_1) \neq \operatorname{lsb}(m_2)$. When the messages returned he can distinguish just by observing the lsb's of $MAC(r_i)\oplus m_i$