doc uigetdir
will let you search for a directory
doc dir
will let you manipulate the information content of a directory, such as the listing of files and filter out stuff you don't want (e.g. '.' and '..' or all subfolders etc).
If you want to read the contents of a file then there are various ways to do that depending what type of file it is, but once you have it all in a string or cell array of strings/chars you can just assign it to the 'String' property of your listbox.
