Start your nodes/instances from the command line of your operating system:
pg_ctl -l $HOME/2ndquadrant_bdr/bdr5598.log -D $HOME/2ndquadrant_bdr/bdr5598 -o "-p 5598" -w start pg_ctl -l $HOME/2ndquadrant_bdr/bdr5599.log -D $HOME/2ndquadrant_bdr/bdr5599 -o "-p 5599" -w start
Each node/instance will start up and then will run in the background. You'll see the following:
waiting for server to start.... done server started
If you see an issue with starting your nodes/instances:
waiting for server to start........ stopped waiting pg_ctl: could not start server
... then take a look at the log files (bdr5598.log or bdr5599.log depending on which one failed to start.) Most likely you already have a PostgreSQL instance running on the target port. It is also possible that your $PATH is not set to point to BDR, so you're trying to use binaries from a different PostgreSQL release that won't have the bdr extension or understand some of the configuration parameters.
Note: Because they were started manually and don't have an init script, these nodes/instances won't start automatically on re-boot.