Is it a PDF? If so, I'd recommend downloading Adobe Reader, which is free.
http://get.adobe.com/reader/
It handles editable PDFs better than Preview, at least in my experience. You may want to download a fresh copy of the application if you can, I've had Preview do weird things to editable PDFs.
If you still can't edit it, you can always overlay text, but it's easier if you don't have to. Preview has annotation tools, you can add text boxes, I believe.