Stupid question, I bought the megaton edition from gog, and am able to play the original episodes plus the "Brith" episode. But I cannot find the installer or files for "Duke it out in D.C." "LIfe's a beach" etc, anywhere. Can anyone point me to where the installer or files are located?

GOG doesn't have the Megaton Edition, only Steam has the Megaton Edition. As far as I know, Megaton Edition is a Steam exclusive, just like Shadow Warrior Redux is.

GOG has the Atomic Edition which is the main campaign (original 3 episodes) plus The Birth episode.

There's also an entry on the wish list for upgrading the GOG version to the Megaton Edition.

http://www.gog.com/wishlist/games/duke_nukem_3d_megaton_edition

The likeliness of megaton happening here is honestly very low (unless the devs either want to port it to Eduke32 or do a huge overhaul of the coding) because Megaton uses Steamworks for multiplayer, therefore absolutely depending on DRM.

You can however, play the addons in this version easily with normal DOSBox if you can get your hands on the GRP files. Honestly if you play them through EDuke32 you get a better quality experience compared to Megaton.