The instructions are a bit out of date - after clicking Add just navigate to the file that you want to add and select it and click 'Add' on the popup and the popup should disappear and the document should then show on the 'Numbers Documents' list on that screen in iTunes.
To then get the document into the Numbers app on your iPad so that you can open it, then follow steps 5 to 7 from http://help.apple.com/numbers/ipad/2.2/#/tan33125c726 :
- Connect your iOS device to your computer, then open iTunes on your computer. After a moment, your iOS device appears on the right side of the iTunes toolbar.
- In iTunes, select your iOS device, then click Apps.
- Scroll to the bottom of the Apps window, then select Numbers (in the Apps list below File Sharing).
- Click Add, select the file, then click Open.
- Open Numbers on your iOS device.
- In the spreadsheet manager, tap
, then tap Copy from iTunes.
- Tap the spreadsheet.
(The spreadsheet manager is where you see a list of the documents that you have in the app, and can either open an existing document or create a new one)