If you build a part in r15 and try to open in r11 you will have data loss that is unavoidable because Catia is not backwards compatible. I mean you can try it. But to my knowledge it won't work.
That's what you must do. You cannot open it any other way, except to export it as IGES, or STEP, if you have the license. (and then open it in the lower version)
To add to what Derek said - you cannot do anything to retain history from a newer version back to an older. If you need history, you'll have to re-create the part in the older version.
The steps for using the Downward Compatible Utility are in the online documentation. You should look it up first, and ask specific questions if you still need help after that.