UnityMol started many years ago as a project on SourceForge. For this historic reason, the primary download site has been the SourceForge UnityMol Files page. A historic archive of UnityMol versions can be found there.
When we transitioned to GitHub, we started to provide UnityMol sourcecode releases and associated binary downloads through the release mechanism. From now on you should be able to get the latest releases from here.
Currently, the latest release to be made public soon is UnityMol version 1.1.4.