I would simply use the TEXT tool. It's in the top-right section of the screen. You click the TEXT tool and your mouse will turn into a cursor. Then point it somewhere in the score close to the spot where you want the text to go and click it. An interactive box will pop open and you can type it what you want the text to say. After doing so, you can use the SELECT tool to click and drag the text wherever you want it.
Guitar position markings are usually in capitalized Roman numerals.