Please see the two previous answers in the thread from xmasbaby and DoninGA.
xmasbaby:
If it is blank on your W-2 then leave it blank on your tax return. There are some people---like teachers--- who do not pay in to Social Security.
DoninGA:
Social Security wages should be in box 3 and the Social Security Taxes withheld should be in box 4 of the W-2.
Contact your employer for either an explanation of why there are no SS wages or taxes withheld or to get a corrected W-2.
