uigetfile replies the path and the file name. You have to use both to address your file:
[filename, pathname] = uigetfile('*.txt');
fid = fopen(fullfile(pathname, filename), 'r+');
Otherwise Matlab searches the file in the current folder. For fopen(name, 'r') Matlab would even search in the complete Matlab path, which might lead to very unexpected effects. But fortunately the write-permissions triggered by 'r+' let fopen be less smart.
