lesser elemental rage useage

when lesser elemental rage is activated, does every attack add a 1d6? or do you add just 1d6 at the end of the damage this round?

the text reads:
"as a swift action, the barbarian can cause her melee attacks to deal an additional 1d6 points of energy damage"

note: This power can only be used once per rage.

The quote continues 'for one round'.

It lasts from when you activate it, to just before the start of your next turn, and applies to all melee attacks made during that period (including Opportunity Attacks).

thank you for clearing that up!