[Paraphrased from the excellent rec.audio.pro FAQ at
http://www.cis.ohio-state.edu/hypertext/faq/usenet/AudioFAQ/pro-audio-faq/faq.h
tml
amongst other places]

The correct way to connect balanced and unbalanced equipment is an audio
balance transformer

To connect an unbalanced output (typically on a phono connector from
some home audio equipment, eg a CD player) to a balanced input (almost
always an XLR connector). Connect the centre pin to pin 2 of the XLR
connector, and the ground ring to pins 1 & 3

To connect a balanced output to an unbalanced input is trickier. If it's
a floating (passive) balanced output you can connect pin 2 of the XLR to
the phono pin and pin 3 to the ring. If it's an active balanced output
then you may be able to XLR pin 2 to the phono pin and pin 1 to the
phono ring, leaving pin 3 unconnected. If that fails try connecting XLR
pin 3 to the phono pin, XLR pin 1 to the ring and leavin pin 2
unconnected. Both of these approaches may well cause distortion or more
noise.