It's hiding in the annotation command.
h = annotation('textbox')
After you've created it, you can use the set command to change things.
set(h,'String','Something or other')
set(h,'Position',[.3 .8 .4 .1])
set(h,'HorizontalAlignment','center')
You can also pass those sets into the annotation command:
annotation('textbox','String','another one','FontSize',24)
