MPC Standalone | Why Aren't My Native Instruments Expansion Packs Displayed?

The reason is likely because you transferred the enclosing folder instead of the expansion pack folder itself to your MPC hard drive.

Please refer to the GIF below, which goes through the parent folders to reveal the correct child folder at the end. This is the folder that needs to be transferred to your MPC hard drive. 

Please also refer to the screenshot below...