computeDFT is a private function in the same directory as the stft.m file. As such it is intended only be called by code in that same directory. If you select the call in that file, right-click on it, and select to open the file MATLAB will look for that function "in the context of stft" which is in the directory that can access the private function.
If you were hoping to call computeDFT in your code that is not possible unless it is in the toolbox directory, and I strongly recommend against you putting your own code in the toolbox directory (or indeed any directory under matlabroot.) This private function is not intended to be called by users directly.