Your wife needs to fill out a SS-5 form and apply at her local Social Security Administration office.
She will be required to show the EAD to the SSA agency to be eligible to receive a SSN.
To apply for a SSN she will need to go to her local SSA office and bring:
1. The SS-5 form downloaded and filled out (they have some at the office if you forget).
2. Passport
3. Valid I-94
4. another form of valid ID (as defined in the SS-5 instructions)
5. A valid EAD
Only persons age 12 and older must apply in person. A derivitive visa beneficiary may elect to have their parent apply on their behalf (they will still need all the required documents). The SSA will most likely need to verify the validity of the EAD and will make a copy to send to the USCIS as part of the SSN processing. Once they verify the EAD she will receive the SSN card in the mail.
You can check on the status of your SSN application and/or often get the SSN in advance by returning in person to the SSA office. If you need your SSN before it arrives in the mail you may ask them for an SSA-7028, Notice to Third Party of Social Security Number Assignments, to be sent to your employer or other interested third party once a SSN has been assigned. Your card, when you receive it, will be marked with the words "VALID FOR WORK ONLY WITH INS AUTHORIZATION" indicating to an employer that you must also have an unexpired EAD to be eligible to work.
For more details, go to the following link on the ssa website:
http://www.ssa.gov/pubs/10002.html#how