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1. Call 1-866-230-5091 any time of day to access our automated account
activation system from the phone number you entered in your sign up.
If you are located outside of the United States, please call our main
Customer Solutions number: +1 (626) 685-5700 during our business hours:
Monday - Friday: 6 a.m.-7 p.m., PST and Saturday: 7 a.m.-4 p.m., PST
2. Enter your account number, found in the confirmation e-mail you
received from Yahoo! Search Marketing or your account
3. Verify your account information as prompted, such as your credit
card information and zip code (Please have your account information
available to complete this step).
If you have a rep, forward them the email and they can usually fix the issue.
If you don't, then you'll need to call Yahoo and input the number. You can also call their general support line, tell them you wish to verify an account, and then just read the the number.
It's a pretty straightforward (although often very annoying step) process.