How to unlock note 3 bootloader : Secure download :Enable

On download mode my note 3 shows SECURE DOWNLOAD : ENABLE in blue colour which I guess means my bootloader is locked . Whenever I flash any custom rom like cyanogen mod or any custom kernel I get stuck on kernel is not seandroid enforcing set warranty bit kernel . I'm currently on stock lollipop rom .

so my question is how to unlock note 3 bootloader so I can flash custom rom (I can flash and boot rom darkera but not cyanogenmod or aosp , aokp rom)

And how to remove SECURE DOWNLOAD : ENABLE from download mode

Please Please help