Jackpot Euros are for use in the Jackpot Arena. When you sign in, a message will appear and tell you what date and time the Jackpot Arena starts. Once you click on the message and enter your bank details, you will have a chance at winning your number of jackpot (GBP, USD, etc.) in real cash. These battles are not to be taken lightly and are very difficult to win. The maximum about that can be won is $10,000 USD.
You can collect Jackpot Dollars in bonus boxes, and also by completing certain quest strings (which also includes acivities such as "Spaceball").