I try to repeat the example shown in: www.limesurvey.org/manual/Expression_Manager
But I don't know where and how I have to insert the formula .
I also don't understand how to use equation manager.
I appreciate your help.

You will find all cited or related demo surveys (ready for importation) in the the folder /docs/demosurveys of your limesurvey package...
Also available in the manual or github