Previous releases of the AMT can be found on Sourceforge download page.
You can get the latest source codes by accessing the GIT repository on Sourceforge. Access is possible by the Web interface or by using an GIT client to access the repository at:
git://amtoolbox.git.sourceforge.net/gitroot/amtoolbox/amtoolbox
To install, first unpack the package to a directory. Then, you will need other toolboxes and packages. The requirements are described in the documentation of amtstart and amtmex of the corresponding version.