Quicksilver only indexes /Applications/ and /Users/yourusername/Applications/ by default.

In order to have access to Xcode and other Developer Tools simply add a new File & Folder Scanner from Quicksilver's Catalog preferences panel and specify com.apple.application as type. Additionally you need to specify a scan depth of 3.

File and Folder Scanner

Once done that, close the Preferences and re-scan your Quicksilver catalog. Then you will be able to launch Xcode the same way you launch other applications.

Quicksilver opening Xcode 4