In this code:
[FileName,PathName] = uigetfile('../*.csv','MultiSelect','off','title','path');
FileName is simply the name of the file you selected. So if the file is not located in the present working directory, fopen will not be able to read it. So it will return an fid of -1, which is an invalid file 'pointer'.
What you need is to supply the full path of the file to fopen. So doing this:
fid = fopen(fullfile(PathName,FileName));
will return a valid file 'pointer'.