how to reboot if macbook freezes

if it freezes during game play you could try a Force quit. Hit Command + Option + Escape, the quit window may or may not show up, but if you hit Return it may be enough to quit the game, provided the whole OS hasn't locked up.

Its not really a good idea to make a habit of shutting down like this as it can cause directory corruption. Its not going to hurt your computer itself, but it could cause problems for your OS. I would get in the habit of running the fsck command after a forced shut down.

Boot holding Command + S keys. Continue holding those keys until you get a black screen with text. Allow that to load and type fsck -yf and hit the return key. When you get the message that "The disk has been repaired" or "The disk appears to be okay" type reboot and hit return.

Altough force shutting down a computer by holding down the on/off buton for a few seconds and then letting it go, is not widely advised, it doesn't seem to do the computer much harm (at least in my experience). I've force shut down my MacBook this way a few times and the computer still works fine when rebooted. Also I rebooted it over christmas when I encountered a problem surrounding blank messages, this way and it solved the problem. I would however ensure all your files are backed up before forcing shutting it down, just be on the safe side.