Where can I find the task manager in Mac OS X Lion?

It's called Activity Monitor. You can find it in this folder: /Applications/Utilities/

A short way to do this is using the cmd + space to open Spotlight and then type in Activity Monitor (you should get the right suggestion very quick). I like this, because it's very fast.

You can also use this to start other apps or open files.

Look for the apple/Mac icon (perhaps top left). There will be an option for "Force Quit." Use that.

There are many ways to find mac task manager. Easiest one is to use spotlight. Just type Activity monitor i.e mac task manager.

If you wish to just force quit an app or process. You can just use this shortcut: Alt + Command (⌘) + Esc

Source: Mac Activity Monitor