To install an expansion that is in .xpn format to your MPC, you first have to install the expansion to the software and then export it to your MPC's external drive.
Installing the Expansion to the MPC Software
To install the .xpn expansion to the software, open up MPC and simply drag the file into the software.
Exporting expansion to MPC Hardware
Navigate to File >Export > Expansion...
Select your MPC's external drive, check off the Expansion that you wish to export, and click Export.