The latest releases will always be available on the Sourceforge download page along with release notes describing what's changed. Just find the latest release and click the "Download" link by the "mrbs" package like this :
Alternatively you can get MRBS via Mercurial, instructions are here. Releases are currently made from the 'default' branch.
$ hg clone -r default http://hg.code.sf.net/p/mrbs/hg-code mrbs-hg-code
For installation instructions please refer to a file called "INSTALL" in the tarball that you download.