Just drag the
/Applications and start it from there.
Xcode used to be installed to
/Developer, but not anymore. It's a self-contained application package now.
According to Apple's release notes,
the simplification afforded by repackaging Xcode 4.3 as a single app bundle eliminates the need for the /Developer directory containing prior versions of Xcode. As a result, the Install Xcode application and the uninstall-devtools command line script are also no longer needed.
Furthermore, the Xcode command line tools are …
not bundled with Xcode 4.3 by default. Instead, they can be installed optionally using the Components tab of the Downloads preferences panel as shown in Figure 3.
So open Xcode.app, then click Xcode » Preferences, or press ⌘,