Latest updates and discussion:
Make sure you have KSP 0.22 or higher. Support for KSP 0.21 is discontinued, use R3.4 if a compatible version is needed. Support for KSP 0.20.2 is discontinued, use R3.2 if a compatible version is needed. Versions below 0.20.2 are incompatible completely, even with older releases.
Double-check whether you are using KSP version 0.22 or higher. Your root KSP folder should already contain GameData and Ships folders. Overwrite them with folders from the archive. Do not, under any circumstances, extract just GameData/B9_Aerospace/Parts folder alone, the mod won't work this way. With a proper installation, you should have multiple new folders in GameData alongside with Squad folder that was there originally. Do not try to move the part folders elsewhere or sort them by category subfolders, that will break resource linking in all configs.
You don't need the Source folder for the mod to work, it contains source code for all the plugins.
How to use
Check example crafts to get the idea of how all the parts can be used, and start building!