Your Eurail Paper Pass must be activated before you can start using it. It must be activated within 11 months of the purchase date.
You can get your Pass activated at a European train station. This service is always free of charge. To do this, simply go to the ticket office of any large train station in one of the participating Eurail countries. A railway official will then activate your Eurail Pass by filling in the start and end date of your Pass. They will also stamp the Pass in order to validate it.