First, play the game and grab HAPPY essence (because it is easy to obtain) or APPLE (because it is the first essence), then save. Exit the game and go to "My Documents\Electronic Arts\Mysims\SaveData#" (where # is a number). Open playerRecord.xml with Notepad (make a backup copy first!) and find the following line:
<happy type="number"> (or <applefruit type="number"> )
In this line you can edit the number of essense (up to 99), i.e:
Apply the changes, start the game and load the savegame.