Not sure how the mac version differs from the pc version, but it's pretty simple on the pc.
Open Excel and make sure you have the "Developer" tab as an option in the ribbon. If you don't add the tab by customizing your ribbon within Excel options.
In the "Developer" tab, there is an "Insert" option (icon with a tool box, hammer, and wrench). Click on that to view the various controls you can insert. A button is one of the options, so click on the "Button" option and then you will be able to insert a button into your sheet the same way you would insert a picture, text box, shape, etc.
To assign a macro, right click on the button, choose "Assign macro". You should be able to figure out the process from there.