I have an HMC6352 and I posted a message about having problems connecting it with my Lilypad. I've tried everything I can think of, including using various pins for I/O, trying different code (Thanks BenF!) and changing USB/Serial ports. I'm stumped. Is is possible that my HMC unit was DOA? Or, is it possible I killed it when attaching the wires to the breakout board I bought from Sparkfun? How can I tell?? I have a multimeter but I have no idea how to use it...
Thanks in advance
There is not a lot you can do with a multimeter. Check that it has voltage on the Vdd pin. Check there is continuity from the ground to the ground of the Arduino. Check that there is the Vdd voltage on the SDI and SCL lines.
After that you need an oscilloscope to check that you are getting a clock signal on the SCL line when you send it data. Then look at the SDI and identify it is pulling down at the end of a byte transfer.
Finally make sure that you are using the correct address (0x42 and not decimal 42).
This is excellent, thanks Mike. You are presuming that I know how to, for example, "Check that there is the Vdd voltage on the SDI and SCL lines" though. Do you mean using the multimeter? I'm sorry, I'm sure it's frustrating dealing with noobs. 
Yes check with the multimeter that on the two I2C lines there is a voltage that is the same as you are using to power the I2C device. So if it is 5V check the signals are pulling up to 5V if you are using 3.3V check it is pulling up to that.
Vcc is the short hand way of saying "what ever power supply you are using"
However you can't get far in electronics without an oscilloscope, even a simple one made with an arduino is much better than nothing.