The Murder on Eridanos DLC will not appear in the Epic Games Store as an Add-On. Simply verify that you own the DLC, whether you purchased the DLC individually or the Expansion Pass, and launch the game.
On the main menu, you should see a green check mark on the panel for the Murder on Eridanos DLC. If you see it, you're good to go! Talk to ADA after having unlocked the Stellar Bay launch pad and you'll be able to make your way to Eridanos.
If you don't see the green check mark, try closing down the Epic Store completely by right-clicking the EGS icon in the Windows tray and selecting "Exit", and then relaunch the application.
If you're still not able to access the DLC after this, please send us a ticket by clicking here.