Change to the Relay Network region closest to your p2pool node:
ExecStart=/bin/java -Xmx100m -jar /PATH/TO/RelayNodeClient.jar public.YOURRELAYREGION.relay.mattcorallo.com 127.0.0.1:8333
Shouldn't this be:
ExecStart=/bin/java -Xmx100m -jar /PATH/TO/RelayNodeClient.jar public.YOURRELAYREGION.relay.mattcorallo.com:8335

No, the above is right running the jar version... I believe it connects to your local bitcoind instance first, and interacts with it, relaying out to his server on 8335/8336. At least that's the way I am running it, and it seems to work.
BTW for those running it on the commandline, don't forget "nohup ... &" if you're not running in the same screen as when you started p2pool.