Krosan Restorer’s activated abilities aren’t mana abilities. They each use the stack and can be responded to. Notably, they can’t be activated while you’re casting a spell or activating another ability. If you want to use Krosan Restorer to add enough mana to pay for a spell or ability, you may tap land(s) you control, untap them with Krosan Restorer, and tap the lands again all before you start casting a spell or activating an ability.