All you have to do is make a layer above the one on which your viewports are placed and add your text and leaders on that layer. Unlike in SketchUp where layer order doesn't matter, in LayOut the order of the layer list is important.
I have my templates set up so the SU viewports go on layers quite low in the list. Dimensions, hatches, labels and other text and dashed lines go on layers higher up in the list.
As to changing the text, it depends upon the OS. On PC you can change the font in the Text Style inspector window. On Mac you can get to the Font window most easily by right clicking on the text.