Hello,
The reason you are getting error is might be because by default that folder is not added in the MATLAB path. You can run line number 82 to 84 from the example and then you can try running below command to open this file.
>> open detectObjectOCV.cpp
You can also use below command to go to the support package root location where you can find this source file.
>> dirFullPath = fullfile(codertarget.robotmanipulator.internal.getSpPkgRootDir,'robotmanipexamples','opencv_mex');
>> cd(dirFullPath)
Please try the mentioned steps and let us know if you are still facing any issues.
Thanks