The following information comes from Wookiepedia, the Star Wars Wikia:

The game has a hidden level which allows the player to take the role of Luke Skywalker during his visit to Bespin in Star Wars: Episode V The Empire Strikes Back. The website that LucasArts released this information on was well and truly hidden with no pages linking to the required files. Discovery of this page spread by word of mouth.

Instructions on how to play it can be found here[web.archive.org]. To add to the problem, the downloaded 'episode.jk' file occasionally downloaded as 'episode.txt' depending on the users PC and browser configuration. Simply renaming 'episode.txt' to 'episode.jk' solved the problem.

In this guide I'll simply repass the original guide, so that the Steam Community can also enjoy it!

1. On your Steam Library, right-click Star Wars Jedi Knight Mysteries of the Sith, go to Properties/Local Files > Browse Local Files...
   
2. If there isn't one already, create a new directory in your MotS directory called "Episode"
   
3. Inside the "Episode" directory, create another directory called "MotS Secret Level"
   
4. Unzip the UNSUPPORTED.ZIP file, which can be found in Resource/Video/
   
5. Copy the "Jkl" and "Cog" directories from UNSUPPORTED.ZIP into the "MotS Secret Level" directory you created on step #3
   
6. Download this file[web.archive.org] and save it on the "MotS Secret Level" directory (don't worry, it's a safe link, I can put up another mirror if you guys want). NOTE: there is a chance the link open as a .txt file. In that case, you have to save it to your computer and then change the file extension from "episode.jk.txt" to "episode.jk".
   
7. You're all set! Run Mysteries of the Sith, select Single Player > New Game > MotS Secret Level to play the level and enjoy the recreation!

Original guide from the LucasArts website via the Internet Archive[web.archive.org].

In this level, you play as Luke Skywalker, having just arrived on Bespin in The Empire Strikes Back. You start off right at the landing platform alongside R2-D2 and Luke's X-Wing.

You start with Luke's lightsaber and a simple blaster pistol. You also have a Bacta Cannister and a light module available. Your Force Powers are Force Jump, Force Pull and Force Bind. Through the level you'll collect Stormtroopers rifles, and a little more to the end, a Rail Detonator and a Concussin Rifle.

You have no objective on the Mission description. All you have to do is walk through the hallways of Bespin and blast some Stormtroopers and Imperial Officers. Eventually you'll find Boba Fett with some Stormtroopers, but he's just a common enemy here, not the awesome yet tough as ♥♥♥♥ boss from Jedi Knight: Dark Forces II. If you are curious enough, with some exploring you can find two secret rooms with some items, and also another landing platform with the Millennium Falcon, where some guys are messing with the hyperdrive just like it is stated in the movie!

The weirdest part is that there's no Darth Vader. Instead, you fight Dark Mara Jade on the Carbonite Chamber, and Dark Kyle Katarn on the bridge. They are VERY hard to beat on a lightsaber battle, but there's another way: they're both completely defenseless against shooting! None of them can't even dodge or defend with their lightsabers, so you can just shoot away! I guess that's because when you fight both of these bosses on the MotS storyline, you can't use firearms on any of them. So the programmers probably didn't even bother to program a blasting defense for them!

Anyway, after defeating Dark Kyle, you can just drop down the pit like in the movie and fall on the Millennium Falcon, and that's it!

I guess you can call this one of the first DLCs ever! And it's free!

Here's a video I found on Youtube showing the whole thing: