The "G4M Classic Doom Launcher" launches Classic Doom for Doom 3. Simply drag it to your Application folder, choose your Doom 3 Application and have fun with Classic Doom.
The mod is Classic Doom for Doom 3, and will feature the first episode of The Ultimate Doom. Simply download the Linux-Version of the mod here and put the "cdoom"-Folder into the Doom 3 Folder.