There's two questions here - one is "when does the player announce the interrupt power?" and "when does the power occur" There's no disputing that the bonus to the attack has to occur mechanically prior to the roll being made so that it can add to the roll. So what we're really asking here is can the player declare it after the triggering roll is made and then retcon the bonus in? My answer is yes - the warlord can declare the power after the roll is physically made. Immediate Interrupts are too valuable to just throw away and the whole point of a Warlord (IMHO) is the ability to say "no, my buddy hit"
I must disagree on your interpretation here. My reading of this power is that the warlord must use this ability after the triggering player declares his attack but before a roll is made. Sfedi already gave a 3 step breakdown of the act that I think is quite helpful but the greatest argument in favor of the pre-roll position is that if Wizards wanted this ability to be used post roll then they would have had the interrupt trigger be "An ally misses with a basic attack, a bull rush, or a charge within 10 squares of you". This would clearly allow the player to determine hit or miss first and then decide to apply the power or not. Since it's an interrupt, it can change a hit to a miss.
As for whether this power is underpowered or overpowered, I can't recall what the rest of the warlock utility 6 powers were like but I think this is still a balanced and useful power for an offensively minded warlord to take. It doesn't provide more healing but a +6 before an attack adds at least a 25% additional chance to hit so the warlord is effectively doing 1/4 of a basic attack's damage for free every encounter. It also combos nicely with commander's strike or a barbarian in the party.