How do you show that the set of incompressible strings is undecidable?

The following paper states:

http://minerva.ufpel.edu.br/~campani/cisst04.pdf

"It is interesting to observe that the problem of determining if
a string is incompressible is an undecidable problem. Suppose
that the set of incompressible strings is enumerable. Let f be
a recursive partial function that enumerates the incompressible
strings. Thus f(n) denotes the first string x0 with complexity
greater than n. Then, C(x0) > n and C(x0) = C(f(n)) · C(n) + c = log n
+ c. So C(x0) > n and C(x0) · log n + c,
for any arbitrary n, which is a contradiction. Observe that log
denotes the base two logarithm.
It means that we cannot formally determine how much
information is necessary to compute a string, this being a
limitation of the formal systems."

sorry i meant to ask something else ( i posted a new question)