In Xcode 4.0, Apple removed support for using custom interface builder palettes provided by IBPlugins in the Builder interface editor. Xibs / Nibs that use such plugins will compile, but they cannot be edited. You will need to install Xcode 3 (in a separate folder so as not to overwrite 4) and use it to edit xib with custom palettes.
This is all in the Xcode 4 release notes. Stay tuned to receive notifications of additional updates on this subject.
Update for Xcode 6
Plugins are no longer needed. See @IBDesignable and @IBInspectable. :-)
Joshua nozzi
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