CASA 4.2.1 for Mac could be downloaded from the developer's website when we last checked. We cannot confirm if there is a free download of this app available. The application lies within Education Tools, more precisely Science.
This free Mac application was originally designed by Associated Universities, Inc. The most popular version among the program users is 4.2. Please check the Mac app with an antivirus before launch as it is downloaded from the developer's website, and we cannot ensure that it is safe.
From the developer:
CASA (Common Astronomy Software Applications) is a comprehensive software package to calibrate, image, and analyze radioastronomical data from interferometers (such as ALMA and EVLA, both shown below) as well as single dish telescopes. The CASA infrastructure consists of a set of C tools bundled together under an iPython interface as a set of data reduction tasks.
You may want to check out more Mac applications, such as Mental Case, The Mysterious Case of Dr. Jekyll and Mr. Hyde or CASA-Intuition, which might be related to CASA.