Muscle SpikerBox Pro is a HID USB device and there is no easy way of integrating it with Matlab. Probably it would be necessary to write C++ or Java that will get the data from Muscle SpikerBox Pro and send it to Matlab. All the details about protocol can be found in our guide:
https://github.com/BackyardBrains/SpikerBoxPro/tree/master/Muscle/documentation
Only easy integration would be to use audio output from Muscle SpikerBox Pro and connect to microphone input on your computer and record EMG data from Matlab as you would record audio from microphone. The downside of that method is that you will be able to record just one EMG channel.