LR doesn’t support tethering the D3100. If you are using another product and that product doesn’t work then I’d contact the makers of that product to troubleshoot. If there is some aspect of LR involved in this, then explain what that is as part of your question.
Here is a list of camera models LR currently tethers to:
http://helpx.adobe.com/lightroom/kb/tethered-camera-support-lightroom-4.html