Ubuntu Linux has been going through some radical changes over the last few versions and a change in the latest version, 12.04, is the removal of Synaptic Package Manager. It's hard to think why this has been removed and it can only be because of a dumbing down of Ubuntu. Software Centre has been improved and Synaptic removed.
I'm all for making Linux easier to use, but Software Centre doesn't have all the software that Synaptic has, so you lose access to a lot of great tools and applications. The first thing you should install after installing Ubuntu 12.04 is Synaptic Package Manager.
Fortunately, installing it is very easy. Just start Software Centre in the launcher on the left or open the Dash and find the icon. Enter Synaptic Package Manager into the search box in Software Centre and install it.
Running Synaptic is easy. Click the button to open the Dash and type 'synaptic' then click the icon. If you know the name of an application you want to install, click Search and search for it. If you just want to browse all the software, click the Sections button on the left, choose a category of software and then explore all the items.
Synaptic Package Manager does not replace Software Centre and Software Centre is the preferred way to install and uninstall software. When you can't find what you want in Software Centre though, just fire up Synaptic and search for it in there. It has a lot more in it.