Thanks, rjhughes. Overwrote the old ones in my mod directory with the new one. But it still doesn't appear to change anything - I tried launching the game again and it's still doing the same thing. Brief black screen and then it drops me back to the desktop, with no warning or error message.
I'll plead guilty to being an extreme noob at modding, but it seems I'm still missing something for this to work... For clarity, I will list what I have done.
1. added the textures, gameinfo and mesh files to their appropriate directories in my mod (Distant Stars for Entrenchment 1.051)
2. edited english.str with the entries for Beta Kol and Marza, right below the entries for the normal ships, and increased the count by two
3. edited entity.manifest in the mod directory to include the Beta Kol and Marza, and increased the count by two
4. I added entries for the ships as well in playertech.entity. Could that be what's giving me trouble?