Typically as mentioned above, the text file will be included in the CTF archive, if it was added as an additional file in deploytool. To access these files, I would consider using the ctfroot command. This command will give you the root directory where the CTF archive is expanded (when run as executable).
For example:
fid = fopen(fullfile(ctfroot,'Name of ExE',filename),'r');
If you want to be able to access this txt file outside of the executable, I would typically not include it in the archive (not add it to deploytool) and provide a full path to the txt file. You can even offer the user the choice to select it using uigetfile.
For example:
[path, fname] = uigetfile('Select file','*.txt');
fid = fopen(fullfile(path,fname),'r');
