You need to activate your Eurail Pass at a train station in Europe within 6 months of the issuing date.
It is not possible to activate the Pass yourself!
How to activate your EuRail Pass:
A railway official at a ticket window or a Eurail Aid Office can activate your Eurail Pass. This can be done at most larger train stations. The official will fill in the first and last date of validity on your Pass. Furthermore the activation box needs to be stamped.
Activation is only possible at train stations in the 25 countries which participate in Eurail. When you arrive by train from a non-Eurail country, it is possible to have your Pass validated in the train by train staff when you enter Eurail territory.
Eurostar from the UK: the Eurail Pass doesn't have to be activated - activate it on your arrival in France.
When activating a EuRail Saver Pass: all travelers need to be present with their passport. If not everyone can be present at the station, one traveler listed on the Pass could also do the activation, but he/she will need to bring the passports of all the travelers mentioned on the Pass.