Hessam - you haven't indicated how you have read (uploaded) the data from file, so I'll assume that you have used something like importdata to read that information into a cell array. Once you have done that, you can pass this array to the popup menu object. If you have created your GUI in GUIDE, then you could do something like
varData = importdata('vars.txt');
set(handles.popupmenu1,'String',varData);
where vars.txt is the name of your text file that contains a single column of variable names, and popupmenu1 is the tag of the popup menu object on your GUI.
