do they just telnet into it? anyone know the login?

seriously though, how do they go about accessing the system?

First, you need a 70m antenna.

http://deepspace.jpl.nasa.gov/dsn/

http://deepspace.jpl.nasa.gov/dsn/images/realtime/latestS.jp...

And, it seems simple, but is not: you need very precise pointing, and very precise ephemeris (http://ssd.jpl.nasa.gov/?horizons) to get the relative velocity to the s/c. You need velocity so you know what the Doppler-shifted frequency of the transmission will be.