What does the yellow light mean?

Red light means overheating... unplug it and let it sit for 1/2 hr then try it again.. tpug

The manual should help with this:
http://www.crane-usa.com/manuals/HumidifierManual2008.pdf

Could be a float problem.
The manual has good instructions for fixing it: http://www.crane-usa.com/manuals/HumidifierManual2008.pdf

Hi, Looks like this should be easy to operate. These are the only directions I found for this humidifier: This Crane humidifier operates with cold tap water. Just fill the 1 gallon tank in the sink (fits most common sized sinks) and switch the unit on. The Cow Humidifier will produce cold mist instantanously. The mist output can be regulated with the control button located at the front.
Hope this helps! Hal

Contact the manufacturer, and the retailer let them know about it and try for a refund.

According to the Crane's Troubleshooting section:
The power light is green but there is no mist being emitted:
- Check that there is water in the tank.
- Check for possible lime deposits on the water level sensor or the nebulizer.
- Make sure that nothing blocks the vapor output slots.
- Check that the fan vent on bottom of humidifier is not blocked.
- Empty the humidifier and rinse the water basin with cool tap water and allow to dry for 15 to 30 min.

This next part may or may not be relevant.
Humidifier power light is staying red and not turning green. Or, Humidifier is working then stops:
- Ensure that water level sensor is assembled properly.
- Clean water float.
- Call service center and request replacement water float.

Barring that, here is the service center contact info:
Dart Warehouse Corporation
Tel: 1-888-599-0952 (Business hours Mon-Fri 9:00 am - 4:00 pm Ct)
Website: http://www.crane-usa.com
1835 Ferry Road
Naperville, Illinois 60563