To add letters and text to your pattern, follow these steps:

1. Click Pattern->Add text from the main menu.
2. Type the text you want to add into the Text field.
3. Optionally choose a font and size.
4. Click the Add button.
5. The text is now in the clipboard. Drag the text to where you want it in your pattern.
6. Click Edit->Paste from the main menu.
7. Click anywhere outside the selection to discard the contents of the clipboard.

There is also an instructional video that shows you how to do it here.