I picked up an Airport Express.  Is this installation as simple as plugging it in?  I'm confused because I already have the airport software installed (thanks to you.)  

_______

You don't need to install software.  Open airport utility (you can find it using spotlight search) then double click on your own router's icon first.  Be sure it is set to "allow network to be extended". (see below)  Next double click on your airport express that should be showing if you've plugged it in.  (usually takes around a minute to show up.  If you hit command + R it will refresh.  Now select the airport express to "extend a network" in the drop down menu and select your own network as the one to extend.  (see below) Apply those settings and the express should go green on the light in about 1 minute.  

There's a little more to it than that like actually naming the device and providing a password to make changes to that hardware but that's the main idea.  Those options are availble under the "base station" tab seen below.