I am expecting that you will get the following:
>> which essai
'essai' not found.
Write down the folder where your essai.m file is located.
Go to the Home tab.
Hit the "Set Path" button in the ENVIRONMENT section.
You probably are not seeing the folder in the Set Path window.
Hit the Add Folder button; browse to the folder that has the essai.m file; and hit "Select Folder"