Welcome! Sometimes when I hook up my Kindle to the usb port on my computer, I get "not recognizable error message". But if I restart my computer then it will usually recognize it. Just some little idiosyncrasies of the two systems not speaking the same language.
Oh, and don't forget to check out Feedbooks.com for hundreds of cool books on many, many different subjects. Download them in the "mobi" format to your computer and transfer via usb to Kindle. They also have an mobile site and you can download directly through your browser like MobileRead.
www.feedbooks.com/mobile/kindle