MrBayes may be downloaded as a pre-compiled executable or in source form.
The next release of MrBayes is currently in preparation (autumn 2018) (see Developer version below). Meanwhile, the current release version of MrBayes is 3.2.6, released November 25, 2015. Pre-compiled executables, as well as the release source may be found on GitHub. You can download executable versions for Windows (32-bit and 64-bit), and Macintosh (64bit) computers, as well as the source code for compilation on Unix machines. The Mac installer includes both the serial and parallel (MPI) version of the program, while the Windows installer only includes the serial version; both include the BEAGLE library (v2.1). The serial version works well for smaller analyses but if you plan to run large analyses using many parallel chains, you should use the MPI version instead. Refer to the User Manual for help with installation of the program.
You can get access to older releases (from release 3.2.0 onwards), by browsing the releases directory on github.
You may also download cutting edge developer version of MrBayes from the Git repository hosted at GitHub. Note that you have to compile the code yourself. Read instructions in INSTALL file in the src directory of the repository for further instructions.