If you need to flush the DNS cache in Mac OS X, be aware that the commands for this have changed through successive OSs, including Lion.
Flushing the cache is sometimes a good idea if you are unable to reach a site that you know is live but keeps returning an error (you can check whether a site is live for others by typing its address here: Down for everyone or just me?).
You can download my free app FastTasks that will do this (and a whole lot more! ) for you without you needing to mess around on the command line. However, if you want to do it manually, then open Terminal (Applications > Utilities > Terminal.app) and use the appropriate command for your operating system:
In Leopard / Snow Leopard
In Lion and Mountain Lion
sudo killall -HUP mDNSResponder
With Lion, you’ll have to follow the command with your Admin password, which will be invisible when you type it.
FastTasks – a utility for common terminal tasks