I use Delta quite a bit to Europe.
You have to list for the international portion in ATL. Lax won't be able to do it for you.
In ATL go to the "E" concourse. This is were most international flights depart from. (ALL Europe anyways...) From the tram, go up the escalator and walk straight to the atrium. On your right hand side you will see the Delta Service counters. That's were you list for international jumpseat. Make sure you give yourself enough time. They have to have you listed 75 minutes prior to the flight. The gate won't even have a gate agent yet... If you miss the 75 mins, the system will not take you... They will require a credit card even though you usually don't get any charges outbound to most of Europe. (You will be charged taxes on the return jumkpseat.) If you are going to a country that requires Delta to charge you taxes, they are usually about 30-40 bucks. Don't cut it short on the time! Sometimes there is a line at the service counter! If needed use the red carpet line and apologize when you get to the counter. After all this is done, make sure you check in at the gate for the jumpseat as usual. (One hour prior) .
The return is just done at the regular check in counters at whatever airport you are at... (The 75 mins applies as well even though they don't know about it. Some European airports are Huge and hard to navigate, so give yourself plenty of time. Since they don't see jumpseaters as often (Even though it is better now), they tend to shuffle you between lines and counters until someone takes care of you...
That's it!!!!!
Enjoy the trip.