The identity plate is accessed from the Edit menu by selecting Identity Plate Setup. This brings up a pop-up window where you can create either a styled text identity plate or use a graphical identity plate. Using text is straightforward - just choose your font and color like in any program. The font size is only important in regards to how it displays at the top of the Lightroom window - when printing you can resize to match your output.
Once you've created the text you like, be sure to save it by clicking on the window next to "Enable Identity Plate" and choose "Save As." A great feature is that Lightroom lets you save as many identity plates as you need that can be accessed at any time directly from the Print, Slideshow and Web modules.