MrBayes may be downloaded as a pre-compiled executable or in source form.
The most recent release version of MrBayes is 3.2.7a, released March 6, 2019. Pre-compiled executables are not yet available for this release, but the release source code may be found on GitHub. You can download executable versions of the previous release for Windows (32-bit and 64-bit), and Macintosh (64-bit) computers.
The macOS 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.
The 3.2.7a source code is available for compilation on Unix machines.
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 the INSTALL file in the source code repository for further instructions.