Question:
When putting the proxy.pac configuration in the NetScaler session policy, windows devices are configured successfully.
Mac devices doesn't. It shows blank area for proxy tested on all browsers. 

Is Proxy PAC file supported for MAC OS?

Answer:
The Proxy PAC file is supported *only* on Windows client in 12.0

An enhancement is already filed.
690852: Proxy PAC support for MacOS

Adding to it fix is added to next release on Firmware 12.1.

Fix is also included in upcoming version 12.0.56+ releases.