How to import a spreadsheet into Numbers from an email attachment?
This screen shot is from Keynote for iPad, but Numbers for iPad is pretty much the same:



When you view the file in Preview mode, the title bar of the document should look pretty similar to the above. Over there on the right is the Open in… button.

There might be a  button that you have to click first before you see the Open in… button.

Other ways to transfer files to your iPad:


Use iTunes file sharing to import iWork documents
Use the iWork.com public beta
Use MobileMe iDisk or a WebDAV server