I have been very clear in the product description about this, but I'd like to reiterate it again here as I keep getting complaints about the tool not working. The tool is indeed working. You just haven't done enough reading to make it work. 

You will need two things to make this tool work:

1. An OpenAI API Key which you can generate here.
2. And API credits to make your API key work. You can top up credits from this page. 

After checking off those two things, simply copy your OpenAI API key and paste it in the settings within the tool and you're good to go.

Does that mean there is an additional cost on top of the one time payment for the tool in order to get it working?

- Correct. You will need to top upyour OpenAI account with api credits so that your api key can work. 

How much would the additional cost be?

- That'll depend on your usage of the tool. As of writing, the minimum amount you can top up from OpenAI is $5. A $5 credit should suffice for a full game or two with normal usage. 

Do the credits expire?
- No, they do not. 

If you need more help, my discord is open.