The path is relative to the git repository I referenced. It contains patches that are applied to our custom PVE kernel, but you can add your own in the same folder.
Keep in mind, building your own kernel is not a trivial thing and can lead to many unexpected errors.
Maybe it's enough to install the packages provided by MPTCP (
https://multipath-tcp.org/pmwiki.php/Users/AptRepository) and not have to compile a custom kernel?