Follow the below steps:
With 13.0 build 41.20:
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
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.