Wildcards like @ are only usuable to perform a find.
Middle extracts a section of the text in the first parameter starting at the postion indicated in the second parameter and returns the number of characters indicated in the third character.
Middle ( "123456789" ; 2 ;3 )
That's the 3 characters from "123456789" starting with the 2nd character.
So if you have scanned this data into a field named ScanField, a Middle function such as this:
Middle ( ScanField ; 17 ; 12 )
Will return up to 12 characters from ScanField, starting with the 17 character.
Hint: whenever a function isn't working for you, look it up in FileMaker help where you can find explanations of each parameter and examples of how it might be used.