where is the ipad backup file stored on a macbook

It's located at ~/Library/Application Support/MobileSync/Backup. Open Finder and go to Go>Go to Folder, copy this into the path box and click Go.

I cant find the library file on my macBook Air or my user file when i search for them! help!

Did you try opening Finder, using the Go>Go to Folder Menu, putting "~/Library/Application Support/MobileSync/Backup" (without the quotes) in the box and clicking Go?

Thanks i didnt have the tilde mark. I found five mobile sync backups but not sure which one is for my iphone (I think one is for an Ipad I no longer have, one is for my ipad 2 and two backups might be for my mini or that could be my iphone5! They each have a string of charaters and numbers but am guesssing that two of the files duplicate the first part of the stream so it might be the same device. I guess if I look at thedocuments saved i will get a sense of which device is backed up by which file

Thanks!

Try opening iTunes on your Mac and going to iTunes>Preferences>Devices. Control-Click on the backup you want on the Backups list and choose Show in Finder.