If you have played LOTRO before but just started playing again (on a new character) then that would likely be why it wasn't activated. Some options are turned off as a default and must be manually re-enabled for every character created. It is indeed the "show target's target" option. I believe it should be located in the UI settings. I don't remember the exact wording of the option, but that's where I'd start.
R5 100 GRD Marevayave - Leader of [U][B]Riddermark[/B][/U]ed For Death
R8 100 MNS Fayah/100 LM Siennah/100 HNT Dinenol/100 RK Dhurik
100 CHN Alachas/85 CPT Dinfaerien/60 BUR Dhax/35 WDN Godoric
R9 100 MNS Fayeh (alt Wilya) - Lonely Mountain Band @ [U][B]Landroval[/B][/U]