Jil,
Use the functions 'Position' and 'TextToNum' in the Validation for your field.
Use Position to make sure the hyphens are in the correct places. Use TextToNum to ensure the other stuff is numeric.
Very very roughly -
( Position(..first hyphen....) = 3) and (Position (...second hyphen...) = 7) and .......
and ( Len(TextToNum(SocSec) = Len(SocSec) - [number of hyphens])
(I can't be specific because I don't know the format of your Social Security number.)
Have a look at the help on both of these functions; they are a bit too long to repeat here.
Have fun.
Paul Cheers,
Paul J.