What error message are you getting?
You probably need to use fullfile to properly construct the path+filename, and then use that to load your file. See the examples on the uigetfile documentation page.
Also, fopen does not load a file. It opens it so you can then read it in. I suggest using readtable. This will load the data into a table, so see the Access Data in a Table page for how to work with this data type.
[dataFile, dataPath] = uigetfile('*.*');
data=fullfile(dataPath,dataFile);
[fid, msg] = readtable(data);
You might also find it helpful to go through the app designer tutorial. Following that, there are several examples you can load and look at and learn from.
