The first $5250 of employer educational assistance is usually tax free. If you got more, and paid tax on it (the income will be include in box 1 of your w-2) that part is considered your money and can be used to claim the credit.
In some cases the excess education expense (the amount above $5250) can be considered a fringe benefit and is not included in your taxable income. Then you cannot claim that part either, as it still tax free.