I got a wireless sensor bar for the Wii and it doesn't work what do I do to make it work?

You have not given enough information to what your problem could be, so I will run through some troubleshooting steps

1. Does the Wireless Sensor Bar have batteries, and have you checked to see if the batteries are low or dead? Make sure you install fresh/new batteries in the Sensor Bar
2. Is the Sensor Bar turned on? Look around the Sensor Bar for a power button or switch, the Sensor Bar is not controlled by the Wii in any way, so it must be turned on and off manually
3. is the Sensor Bar located properly? Make sure the front of the sensor bar is facing you, make sure nothing is blocking the view, and make sure it is not turned to the side
4. Is the Sensor Bar doing anything when you turn it on? Is there a light? There should be a light indicating if the sensor bar is on or not, if there is no light, get a digital camera or camcorder and look at the sensor bar through the viewfinder, you should see something glowing on both sides of the Sensor Bar, these are IR LEDs, IR light can not be seen with the naked eye, and requires a camera in order to see the glow of the light
5.check if the sensor bar is plugged into the wii

If none of this works, it is possible that you have a broken/shorted sensor bar and you will need to replace it, candles will also work in it's place for a temporary sensor bar, or you can use flashlights (incandescent bulbs required, LED and florescent will not work)