You can find it under the Matrex Unstable download section in SourceForge.
The changes from version 1.2 are listed under the Matrex File Release Notes and Changelog page in SourceForge.
The idea is to make sure that if there are bugs in Matrex 1.3 they get fixed before the final version is released.
So please give a try to this version and, if you find bugs, use the tracker page in Sourceforge to report them.
The pre-release version includes only the setup for the generic architecture, which means that when you start Matrex the first time it is supposed to download the SWT library for your PC.
After that Matrex stops, so you need to start it once again to use it.
The download of SWT is a new feature, so if you have problems with it please use the tracker page to report them.