[zgrssd]

I noticed that you are registering the event before you defined the handler. So right now your code is registering nil as a event handler - not your code.
Edit: Actually it is registering whatever happens to be in the global variable "MyEventOnHit". I just asumed it would be nil.

This would be the proper order:

--Declare the handler
local function MyEventOnHit( eventCode , result , isError , abilityName, abilityGraphic, abilityActionSlotType, sourceName, sourceType, targetName, targetType, hitValue, powerType, damageType, log )
    lblCounter:SetText(sourceName..":"..abilityName)
end
 
--Register then handler with the event
EVENT_MANAGER:RegisterForEvent( "tlw" , EVENT_COMBAT_EVENT , MyEventOnHit )

Also, it could be that a second event firing directly after the NPC event is overwriting the label. I would use d and .. to output the data into chat.

--Declare the handler
local function MyEventOnHit(...)
    --takes any amount of parameters and feeds them to d()
    d(...)
end
 
EVENT_MANAGER:RegisterForEvent( "tlw" , EVENT_COMBAT_EVENT , MyEventOnHit )