Root is stored in afs at:

/afs/cern.ch/sw/lcg/app/releases/ROOT/

select version and architecture (adapt first line)

and set the following environment variables (code for bash):

export ROOTSYS=/afs/cern.ch/sw/lcg/app/releases/ROOT/5.27.06/x86_64-slc5-gcc43-dbg/root #SLC 5
export ROOTSYS=/afs/cern.ch/sw/lcg/app/releases/ROOT/5.27.06/slc4_ia32_gcc34/root #SUSE 10
export PATH=$ROOTSYS/bin:$PYTHONDIR/bin:$PATH
export LD_LIBRARY_PATH=$ROOTSYS/lib:$PYTHONDIR/lib:$LD_LIBRARY_PATH
export PYTHONPATH=$ROOTSYS/lib:$PYTHONPATH