You will need your employer's tax identification number, as listed in box "B" on your W-2 form, to download your W-2 information to your tax return in TurboTax. If you do not have that, your employer may be able to provide you with it. Also, it would normally be the same as the number used in the previous year.
If you cannot obtain the ID number, than you will have to request a copy of the form from your employer.