I edited a project in Premiere CC and my client owns Premiere CS 5.5, how can I save the project file so he can use it too?
Short answer: You can't. Unfortunately, there is no way to save a project so that it can be used in older versions of the program. That's a major downside of Premiere Pro, especially since this is possible with most other programs of the Adobe Creative Cloud (InDesign, Illustrator, Photoshop, ...). But at the moment, you can't do that with Premiere Pro.
You could use EDL files (Editing decision list). From Wikipedia:
An edit decision list or EDL is used in the post-production process of film editing and video editing. The list contains an ordered list of reel and timecode data representing where each video clip can be obtained in order to conform the final cut.
However, only the order and timecodes of the clips in your sequence will be carried over. All effects and other settings you already put in place will be lost.