First of all:

1. You need to do the following steps even if you have a paid ChatGPT Plus subscription. To get OMATA AI to work we need a developer account which is independent from ChatGPT Plus. They are two separate things.

2. A developer account is for free. But there will be costs when using OMATA AI via your API key. From experience these can be a few dollars up to $10 per month. It really depends. Check the pricing table here: https://openai.com/api/pricing

Steps

1. Create a developer account at OpenAI

2. Add a payment method and buy some credits

3. Generate an API key

4. Enter the key in OMATA AI plugin

1. Create a developer account at OpenAI

Open this page and create an account or log in if you already have one: http://platform.openai.com

2. Add a payment method and buy some credits

You will need to add a payment method (e.g. credit card). And also you will need to buy at least $5 worth of credits (how to add credits by OpenAI). Otherwise OpenAI will deny your requests.
https://platform.openai.com/settings/organization/billing/payment-methods

3. Generate an API key

Navigate to this page: https://platform.openai.com/api-keys

Create a key and save it somewhere as you will not be shown it again a second time. If you need a new one then just create a new one.

Create a new API key and give it a name.

Copy the API key and enter it in OMATA AI.

4. Enter the API key in OMATA AI plugin

In the setup process or on the settings page change / add the API key.