The MB-System™ home page is at

http://www.ldeo.columbia.edu/MB-System/MB-System.intro.html

and the software package can be downloaded from

ftp://ftp.ldeo.columbia.edu/pub/mbsystem

Typically, you will find the latest full release of MB-System™ and several beta versions containing bug fixes and incremental increases in functionality. In general it is usually safe and recommended to get the latest beta version of the software, rather than the last full version. Beta versions often contain considerable increases in functionality, while full versions are issued with much reluctance and may be more than a year old.

Compressed and tar'd packages might have a naming convention similar to

MB-System.5.0.beta31.tar.gz

Be sure to get a copy of the associated README. And you'll also want a copy of the Levitus temperature and salinity database, which you'll find in "annual.gz" or "annual.Z".