Select the text of graphic you want to link to your email address. From the properties palette, select link. At the top is the URL which by default starts http:// Click on the dropdown arrow and scroll down to mailto: to select it. In the right hand part type your email address.
Or simply delete http:// from the left-hand box and type mailto:firstname.lastname@example.org in the right-hand box. Put something in the asset name field too like My Email then next time you link to it you can just choose the asset name from the list of assets