Use Xperl instead of whatever "Lui" uses :P
Just makes sure to enable the XPerl_RaidHelper sub-addon for this functionality.

I'm a bit confused. "attacking my target" and "attacking the same target as the tank" are sort of two different things.

"attacking the same target as the tank" is easy, you just target your tank and pretty much any unit/raid frames addon will show you "targetoftarget".

For example, the new Blizzard raid frames (which can be used in 5mans as well), has an option to show the tank(s) separately and show the tank's target as well.

Showing things "attacking my target" is trickier because many things can be attacking a your target (whether it be a player or mob). So you'd really need the addon to show a list of things attacking your target. I'm not aware of an addon that shows such a list.

Theres an even better solution that alot of exceptional players use. a /assist macro.

Make a macro to /assist your focus target, focus the tank, and keybind your macro. Don't ever "look" to see if you are on the right target, just hit your keybind and be guaranteed you are on the right target.

(personal preference really, but I use the ~ key)

What he is referring to is an option that X-Perl had to show you how many players were currently targeting your target. Example: I am tanking a trash pack with another tank, and I see that 7 people are targeting my target. This would indicate that the OT and both healers are targeting something else (10 man), which is exactly what you want to see. Conversely, if I was the MT and saw only 3 people attacking my target that would be a time to remind the group to focus on the correct target.