In order to use other devices and software with your TiVo, you'll need what the company calls a "Media Access Key". This number allows you to pair TiVo Desktop on one of your PCs as well as make use of the TiVo iPad app.
Finding your key isn't difficult, but it helps to know where to look. Simply head to your TiVo main menu. From there you'll key down to "Messages & Settings". Here you'll find several selections but we're interested in "Account & System Information". Scroll down and press your OK key. On this menu you'll see a selection for "Media Access Key". That's it! You can now take down the key and use it for whatever pairing you might need to complete.
As an alternative, you can also find your Media Access Key by logging into TiVo account at TiVo.com and clicking the "Media Access Key" link on the right side of the screen.
You'll only need your access key for certain things so there's little need to keep it safe. You can always find it in both of these locations.