Zvezda is the third ISS module in Orbit. You can add it to your own space station scenarios or launch it with proton (Pack Baïkonour Pad 24 + Proton UR500"" by Papyref needed. You can find this addon on http://orbiter.mustard-fr.com). Need spacecraft3 (included).
Included Soyuz Dock Adaptator (SDA)
Enjoy !