If I want to play a card with hidden that has an additional cost to enter ready when it's played, if the additional cost was paid, when do I have to pay the additional cost, when I hide the card or when I reveal it?

When you play a card with the Hidden keyword from its facedown position, you pay any additional costs at the time you play the card (when you reveal/flip it), not when you initially hide it.

Here is the breakdown of how this works:

1. Hiding the card: You pay the 1 Power cost to place the card facedown. This is the only cost associated with the "Hide" action.
2. Playing from Hidden: When you later choose to play the card from its facedown position, you are performing the action of "Playing a card." Even though the Hidden keyword allows you to ignore the card's base Energy and Power costs, you must still pay any additional costs associated with playing that card (such as "As an additional cost to play me...").

According to Rule 353.2 and Rule 354.2, additional costs are determined and paid during the process of playing a card, after it has been placed on the chain but before it resolves. Because playing a card from Hidden is a form of playing a card, these standard rules for paying costs apply at the moment you reveal and play it.