Insert script after the body tag

Hello everybody

I’m sure this is pretty easy to do, but it’s not working for me, so I need help:

I need to insert a script just after the body tag, it’s a “peel ad”. What files should I edit and what should I do?

I’m using Mystique Theme, wordpress 3.7.1

Thanks

[Moderator Note: No bumping. If it’s that urgent, consider hiring someone.]