Carefully check the qualifications. If you are eligible, TurboTax will add it to your tax return. Be aware that of the income limits. Unless you are claiming a child, full-time students under 24 are generally not allowed to take it, or anyone under the age of 18, and you can't be a dependent on someone else's tax return.
If you do believe you qualify, go back to the Federal > Deductions & Credits > You and Your Family > Earned Income Credit section of your return. Answer every question in there. If you do not qualify for the credit, TurboTax will show a screen It turns out you don't qualify for this credit along with an explanation of why not.
@Zypher
