While the SpaceX mod is the most likely candidate, "KK" can occasionally refer to other niche projects:

The mod is designed to bring highly detailed, functional replicas of SpaceX hardware into the Kerbal Space Program universe. It is a staple for players who want to simulate modern spaceflight missions. Key Components Included:

Early and updated iterations of SpaceX’s next-generation Mars vehicle.

Extract the contents of kk.7z directly into the GameData folder.

Full-scale models with realistic engine clusters and stage separation.

Rarely, custom user-made installers for codec packs use short abbreviations.