The LCDD geometry needs to be dumped to plain GDML to be loaded by ROOT.
Before using ROOT, the environment needs to be setup.
export ROOTSYS=/my/root/dir cd $ROOTSYS . $ROOTSYS/bin/thisroot.sh
Assuming that there is a file called test.gdml in the current directory, a script similar to the following will load a plain GDML file into ROOT.
TGeoManager::Import("test.gdml");
gGeoManager->GetTopVolume()->Draw("ogl");
This command should display the ROOT OpenGL viewer with the geometry from the test.gdml file.