OnBlockActivated

@Override
    public boolean onBlockActivated(World worldIn, BlockPos pos, IBlockState state, EntityPlayer playerIn, EnumFacing side, float hitX, float hitY, float hitZ) {
        if(!worldIn.isRemote){
            playerIn.openGui(EnergyTools.instance, CommonProxy.GUI_TELEPORTER, worldIn, pos.getX(), pos.getY(), pos.getZ());
        }

        // I want to send a message to the player when he opens the container. <------------!
        /** Tried it without the !   Am starting to get confused -- Noticed playerIn is instanceof EntityPlayerMP with ! */
        if(!worldIn.isRemote){
            TeleporterTE te = (TeleporterTE) worldIn.getTileEntity(pos);
            if(te != null){
                PacketDispatcher.sendTo(new SyncTeleporterContents(te.getCurrentEnergy()), (EntityPlayerMP)playerIn);
            }
        }

        return true;

    }