First, play the game and grab HAPPY essence (because it is easy to obtain) or APPLE (because it is the first essence), then save. Go to My Documents folder -> Electronic Arts -> Mysims -> SaveData1 (can be 1,2,3 according the savegame number). Open playerRecord.xml with a text editor such as Notepad.
Find the line:
<happy type="number"> (or <applefruit type="number"> )
In this line you can edit the number of essense (up to 99) for example:
(supplied by: ntavsan35)