There are several different ways but the ways that do not involve editing the HTML code are to create a new button in the menu editor and choosing "email" in the link-to section. Enter the email to which you wish to have it send in the field at the bottom.
Similarly you can create a hyperlink by highlighting text and choosing the button that looks like a chain link on the top of the editor. After clicking this you should see a section labelled "what type of link do you want to create" and you would choose "send an email" from here and, as before, enter the email in the field toward the bottom of the window.
