When I run the game with Nexerelin enabled, the game crashes and a error message pops up (see below). I'm not sure what file is missing or what I should add to fix this. Any help would be appreciated.
Starsector 0.9.1a is no longer supported, although you can get the last version of Nexerelin for it here.
I'd recommend updating to the latest version of Starsector (0.95.1a-RC6), available here. (Note that saves from 0.9.1 are not compatible with 0.95 and later)