Quite simply, you don't. At least my version of iPhoto has no text feature. You will need to use another editor for adding text. iPhoto is a VERY limited, basic editor, not even in the same universe as something like Photoshop or even many of the other editors available.
Here is a list that might be of help to you:
Here is a list of free sites and programs:
http://www.picnik.com/ - - - Online editing
http://www.lunapic.com/editor/ - - - Online editing
http://www.gimp.org/ - - - Free program
http://picasa.google.com/ - - - Free program
http://fotoflexer.com/ - - - Online editing
http://photofiltre.en.softonic.com/ - - - Free program
https://www.photoshop.com/express/landin... - - - Online editing
http://www.getpaint.net/ - - - Free program
http://pixenate.com/ - - - Online editing
http://www.splashup.com/ - - - Online editing
steve