I downloaded it from the extras but I don't see anything on the home page indicating that it exists. I already had an active game going when I downloaded it. Will it add it to that game or do I need to start a new one?

If on galaxy go to(next to play) Manage intall/configure/ theres a box to check. It will install if it's checked. If it's already checked it's installed.

Not sure about that

If it installed - it'll be accessible from old saves. Also main screen background will be changed.

When you install Expedition you get completely new starting screen with sea theme.

Thanks, everyone. It is installed now and I have the new "sea-themed" home screen. I'm in Foundry and I think I recall that Expedition launches from Core City, which I haven't gotten to yet. But it sounds like I'll be able to make use of the DLC when I'm ready. I appreciate all of the replies.