ADC (version below 13.0)  
Error while downloading plugin: 
"Forbidden you don't have permission to access /vpns/scripts/mac/Citrix_Access_Gateway.dmg on this server".

Fixed with 13.0 build 41.20  where the Citrix Access gateway plugin file for MAC OS X is present under /var/netscaler/gui/vpns/scripts/mac directory despite upgrading through CLI with or without creating a director under /var/nsinstall. 

Follow the below steps:

With 13.0 build 41.20:
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1) Copy 13.0 build 41.20 package to /var/nsinstall via CLI.
2) Extracted the package directly under /var/nsinstall ( NOTE: No need to create any directory under /var/nsinstall to save Citrix ADC firmware package)
3) Ran ./installns

RESULT: Access the NetScaler through GUI > Download Tab > Clicked on “ Download Citrix Gateway Plug-in for Mac OS X” or “Download Citrix Gateway Plug-in for Windows” > Should be able to download the files

MAC or Vista Folder found under : / var/netscaler/gui/vpns/scripts/

Work-Around for build blow 13.0
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Upgrade or Downgrade through CLI by creating a folder under /var/nsinstall or upgrade or downgrade Citrix ADC firmware prior to 13.0 via GUI.
 

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Problem Cause

Known Issue.

If an administrator upgrades or downgrades Citrix ADC through CLI without creating a directory under /var/nsinstall, then there is no mac directory getting created under ( /var/netscaler/gui/vpns/scripts ). Hence, they were getting Forbidden error message.