I believe this answer will answer your question. To quote the relevant part:
To enable te needed `atrun` daemon, as `man atrun` says, execute: launchctl load -w /System/Library/LaunchDaemons/com.apple.atrun.plist as root (e.g. via `sudo`). Once you've done that, `/var/at/` will be the key directory (though the simplest way is to use `at` as root -- e.g., once again, by `sudo`!-).