Just to clarify:
p = .48 is the probability that an investor has exchange-traded funds in their portfolio.
p =.52 is the probability that an investor does not have exchange-traded funds in their portfolio.
Thus, if you want to find the probability that at most 11 people have exchange funds in their portfolio, you would use:
binomcdf(25, 0.48, 11).
The parameters are (trials, prob of success, max # of successes)
Your answer would have worked if the last parameter meant "at least k successes". But it means -at most- k successes.