We currently do not have a working recipe to install Flair with HomeBrew.
We recommend using MacPorts instead. Please follow the instructions here, specifically the sections for Mac OSX and Compile from source.
Since you are using HomeBrew for other purpose, when using FLUKA/Flair, it’s advisable to define a dedicated environment in your command shell to ensure that the MacPorts binary directories (typically, /opt/local/bin:/opt/local/sbin by default) are listed first in the PATH environment variable.