The software is available for Mac, but there is no Sierra support at this time. I believe they will update it since the requirements specified that it's compatible with 10.10 and 10.11 and these versions are heavily used nowadays.
Mac OS X 10.10 (Yosemite)
10.11 (El Capitan)
You just have to check back later for an updated version of this software. Use the following link to access the releases page: http://www.corp.att.com/agnc/mac/