Equations in docbook not working

Hi i have downloaded a free ebook : 
https://github.com/sonatype/m2eclipse-book

because i'm learning how to write a book.

I need to write equations so i simply added this example 

http://www.docbook.org/tdg/en/html/equation.html

to a xml file of the free ebook. but it does't wort at all.

Can somebody help ?

thanks