How do you know the frequency of LSU? Are you referring to real-world charts? Chances are that the frequency or even the availability of LSU has changed some time over the last years, and X-Plane doesn´t have the VOR or it has a different frequency.
Check the Local Map to see the VOR - if it´s not there you can add it (edit button) or better yet update your whole navdatabase with a new set (available at navigraph, for example).
Jan