If you're getting a checkerboard pattern in your renders then Lightwave is running in 'Discovery' (ie: Demo) mode. There's a number of reasons for this, either you've got hold of a copy without the dongle (in which case it assumes that its a demo copy), or your dongle drivers aren't installed correctly, or your dongle is fried.
If its a case of the dongle drivers not being installed correctly, then surf over to the Rainbow technologies website to get the latest drivers (
www.rainbow.com). If you're reinstalling, make sure the dongle isn't inserted into the port until
after the driver setup is complete otherwise it won't install correctly.