Mrbayes may be downloaded as a pre-compiled executable or in source form.
The current 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: 3.2.0, 3.2.1, 3.2.2, 3.2.3, 3.2.4, 3.2.5
In short, just browse the tags of the MrBayes GitHub repository.
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.
A great aid for resolving problems with MrBayes is the MrBayes mailing list. If you have a question about MrBayes you can ask it by posting the question to the mailing list. To post on this list you have to be subscribed to it. However, the list archive is accessible to everyone.
The list will also be used by the MrBayes authors for announcements of new versions or bugfixes. We, therefore, recommend that you subscribe to the list if you are a regular MrBayes user.