First, we will operate on the assumption that you have the expansions installed into a folder called "Expansions" on the root of a removable storage device, connected to your MPC as recommended. Note: This will not work if the expansions were somehow loaded onto the internal "factory drive," 

Akai Pro MPC 2 Software | How to Install and Use MPC Expansions : Akai Professional 

Once you have identified the removable storage device to which the expansions are installed, be sure that it is connected to the MPC. Then, put the MPC into controller mode. Once you do so, the MPC will act as a USB hub (in addition to being an audio interface and controller for the MPC Software). Now you can

Click on the browser icon outlined in red in the bottom right of the picture below and use the file browser outlined in green to navigate to the "expansions" folder on the root of your storage device. Here you will find your expansions!  
Note: be sure that the filter outlined in blue is set to "All" or "Prog" to ensure that you can see the programs in your expansions.

You can accomplish this by navigating using the MPC GUI. Press Browser>Places and find the location of your expansions.