Go to this website.
Click on whichever model you have of the two. On the next page, look for the Downloads tab near the top. Click it, then on the next page, go towards the bottom and choose your OS. It should start downloading after that (not sure if you will have to click on a EULA or something afterwards, but you might). If you're using Windows 7, you won't need a driver for the keyboard, but you might have to work around this if it's not working. Has it ever worked before? Please let us know if you need any further help.