Unlike TVs, AVRs, etc. there is no discrete IR code that puts an IR Receiver equipped Mac to sleep.

Instead, if you only use OS X' built-in Apple Remote support, a long press of the Play/Pause (or "Select", respectively - depending on model) button will put your Mac to sleep.

That's fine if you're fine with the very limited functionality that OS X built-in Apple Remote support offers. But it's a waste of one of only 12/14 button codes if you want to do more with your remote. That's why, in Remote Buddy, a long press of f.ex. the Play/Pause or Select button is often used to offer additional functionality (most frequently fullscreen mode on/off action are assigned to it by default).

If you're ready to sacrifice a button code for the sole purpose of putting your Mac to sleep - and you want your Mac to go to sleep for a long press of the Play/Pause or Select button, go to Prefs > Mapping > Global mapping and assign the action "Put computer to sleep" to the "Select (held)" or "Play/Pause (held)" entry respectively. Please note that - since you essentially hardwire a button to a particular action - this makes this button code unavailable for configuration and use on a per-application basis.

Best regards, 
Felix Schwarz