The "Secure Empty Trash" button in the finder menu was removed upon the release of El Capitan. To Secure Empty Trash, you must clear the trash through Terminal.
Mac OS 10.11
- Firstly, launch Terminal. To do this, press command and spacebar simultaneously, then type in "Terminal".
- In Terminal, type in "cd ~/.Trash" and press enter. Then type in "ls" (lower case Ls) and press enter again. Open your trash folder by clicking on the Trash icon in your doc. If the files in your Terminal window match the files in your trash (which they should), you may continue. There is no recovering these files after this step. Be absolutely certain you want to delete these files.
- Type in "srm -rv ~/.Trash", and press enter. This process will take some time to complete. Some files may be "protected". If you are sure you'd like to remove all the files in the trash, then enter "y" in the "are you sure?" prompt, and press enter.