How can I prove the languages of incompressible words is undecidable?
Consider the word $w_n$ that is the output of $M'$ given input $n$. 

Note that description of $w_n$ is the description of $M'$, whose length is some constant $c$, plus the description of $n$, whose length is $O(\log n)$ since we can express $n$ in the binary representation. So $K(w_n)\le c + O(\log n)$. If $n$ is large enough, we get
$$K(w_n)\lt n = |w_n|.$$

That is a contradiction since, as you have noted, $w_n$ should be incompressible.