If you have correctly installed the MPTCP-kernel and configured the routing tables as needed, you can start using MPTCP
This server (multipath-tcp.org) is running the latest build of our kernel. Thus, every time you connect to this server you will be using MPTCP.
Download some files http://multipath-tcp.org/snapshots.
You can also visit http://amiusingmptcp.de to see how well your network supports MPTCP on different port-numbers.