- What version of Xcode are you using?
- Are you single-clicking or double-clicking the xib file? In Xcode 3 you must double-click the file to open it in Interface Builder.
- If you double-click the xib file and nothing happens, does the file open if you launch Interface Builder, then double-click the xib file in Xcode?
With Xcode 3.2.6 after fresh installation it seems that you need to start Interface Builder manually for once.
When Xcode was installed into default location you can find in fromMac HD/Developer/Applications.
After I once opened Interface Builder manually it started to open *.xib file from Xcode when double clicking.