There's not a "way" to get ungrounded. If you didn't want to get grounded, you shouldn't have stolen you step-mother's iPod in the first place. Just learn from your mistake, and don't do it again, simple as that
What do you think would happen when they found out that you had taken your step mother's iPod. It will take them awhile before they can trust you again for right now there is nothing that you can do to get ungrounded. How long would it take you to trust a family member if they took something of yours if they did not ask you first. It might have hurt them that they trusted you and you broke that trust by doing what you did.
A suggestion- Continue to be grounded, because you deserve it. Don't complain about it, and be glad she didn't report your theft to the police. Use some of this time to sit down and write her a letter of apology. Not a short letter, but one that comes from your heart. Tell why you were tempted to steal it, how it made you feel, how sorry you are, and what you're going to do in the future as far as stealing goes. Give it to her and give her a hug, let her read it, then go back to your room to continue your punishment. You may not get off the hook, but it WILL help.