In Inkscape, to center object, you have to use Object > Align and Distribute dialog. It allows you to align object relative to other objects, to the page, etc. You can also distribute space between your objects. Here is a video showing how to center an object.
About the author
Xuan Ngo is the founder of OpenWritings.net. He currently lives in Montreal, Canada. He loves to write about programming and open source subjects.