You need to provide more information for anyone to help you. What version of Xcode are you using? This question is important because Xcode 4 changed the way you open xib files. In Xcode 4 selecting a xib file from the project navigator opens the xib file in Xcode so you can edit it. In earlier versions of Xcode, double-clicking the xib file from the Groups and Files list opens the xib file in Interface Builder.
If you create a new Xcode project, are you able to open the xib file? If so, it means the xib file you were trying to open is damaged. The fact that the Finder says it's a Unix executable is a sign something's wrong with the file.