Here is a good article about how to setup a wifi dongle on your Raspberry Pi.
I used the terminal version of wicd – wicd-curses over SSH to set-up my EdiMax EW-781Un as I don’t have a screen connected to my smart-cam simulator. You don’t have to install any other software as a driver for this hardware is bundled in the Rpi build.
The wicd-curses UI is a little confusing so be careful what you press on in there as you can easily disconnect your existing wired network and leave yourself with no network connection (as I did!), if this happens you may have to power cycle your pi to get a network connection back.
In the UI you should see a list of the available WiFi networks, use the up/down arrows to highlight the one that you want to connect to and then press the right arrow key (donated as -> in wicd-curses) to get to the configuration screen where you can set-up the WiFi key etc.
Once you have configured the connection you can connect by pressing C in the main screen, when you connect to WiFi you will be probably dumped off the wired network connection so you will have to SSH in again on the new IP address that was assigned to the WiFi adaptor.