Hi
In order to keep the interface cleaner, in Excel for Mac the button is on the Ribbon. Click the Add Chart Element button on the Chart Design tab of the Ribbon for the options you are looking for.
My personal preference is that I don't like the button attached to the chart. However, many Office for Mac users come from Windows and are used to it being there.
Microsoft has a forum devoted to feature requests and interface design suggestions. I looked through the forum and didn't see a request to have the Elements button display, so you may wish to start a new request. Here's a link to the forum.