The ability reads Interrupt: Negate the action's Melee or Ranged Attacks.
When should I play the interrupt, upon the announcement of the melee/ranged attack, orupon targeting of my personality?
The Turtle's Shell
A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
When should I play the interrupt, upon the announcement of the melee/ranged attack, orupon targeting of my personality?
Technically neither.
All Interrupts to an action are played after all its costs are paid (if any), just before its effects resolve; so, visually speaking, right at the bolded colon.