ATOM supports AAX Native, AU, VST2, and VST3 plugins. It is supported by Pro Tools 10.3.10 or higher, along with any VST/VST3/AU-compatible host running on a supported operating system. 

Bear in mind your host DAW must be running in 64 bit to use Atom. 32 Bit is an extremely outdated operating mode, and many modern daws have dropped 32 bit support entirely (like Ableton for Ableton 10), so we have done the same on OSX. 

There is a 32 bit port that can be used potentially on Windows however. Such a port is not possible on Mac because Apple has dropped the ability for developers to make 32 bit products:
​
​"Additionally, starting with OS X Lion, only 64-bit Intel Macs are supported, so software that specifically depends on new features in OS X 10.7 or newer will only run on 64-bit processors and therefore does not require 32-bit/64-bit fat binaries."

Thus any operating system on Mac older than OS X Lion cannot use Atom unfortunately.