Secure delete removed in OSX 10.11
It was deliberately removed by Apple, so can't be renabled. There are work-arounds, eg. using srm on the command-line, or third party secure delete products.

From: https://support.apple.com/en-us/HT205267

Finder

Available for: Mac OS X v10.6.8 and later

Impact: The "Secure Empty Trash" feature may not securely delete files placed in the Trash

Description: An issue existed in guaranteeing secure deletion of Trash files on some systems, such as those with flash storage. This issue was addressed by removing the "Secure Empty Trash" option.

CVE-ID

CVE-2015-5901 : Apple