How to install MPTCP on RPi 2 & 3 is outlined at https://technofaq.org/posts/2018/09/how-to-compile-mptcp-linux-kernel-on-raspberry-pi-2-and-raspberry-pi-3-on-archlinux-arm/.
An older tutorial on how to install MPTCP on the RPI has been posted on the myPl@yground blog at http://mytechpg.blogspot.it/2016/07/mptcp-raspberrry-pi-and-yocto.html.