You need to travel to your Spore C&C directory (which is normally C:\Program Files (x86)\Steam\Steamapps\Common\Spore\BP1Content\), then you need to copy the file named "BoosterPack_01" to the Data folder (C:\Program Files (x86)\Steam\Steamapps\Common\Spore\Data\).

After that, C&C parts should appear in your game. Enjoy!

If you're one of the people who don't have a BP1content folder, something must be wrong.

First, make sure you have C&C installed.

If you have C&C installed, but there is no BP1content folder, you need to verify the game's files.
To do this, right-click on Spore: Creepy and Cute parts in Steam, and select "Properties".
Then, go to the "Local files" tab, and click on "Verify integrify of game files".

Steam should download C&C's files, and then you can fix C&C's parts not appearing. (if they don't work for you)