with the monitor connected, go to System Preferences > Display
You should see three options; Display, Arrangement, Color.
Choose Arrangement and just under the representation of your two monitors you will see a check box marked "Mirror Displays"
Just launch your stuff, move it where you want it and have at it
Hi, I'm fairly new to macs and would really appreciate a little help if poss. I recently bought a macbook pro and I love it, however as I edit photographs I need a bigger screen so I added my acer monitor. I can't figure out how to have, say for example, adobe bridge on the macbook screen and photoshop on the monitor and work between the two. Is it even possible?, I'm assuming it is. The best I've managed to is to mirror the screens which is good but not quite what i was after. Can anyone help me???