Hi,
This plugin was created around 7 years ago.
It seems the developer documentation on your link is for a newer API.
So I'm not sure of the situation.
We didn't get much feedback on this plugin in the past few years, so it might be that Intuit doesn't support the API used by this plugin, or maybe it only supports it for existing merchants already using it.
The first thing would be to check with a tech support at Intuit about the API using the entry point transactions.innovativegateway.com/servlet/com.gateway.aai.Aai (which is what this plugin uses).
If it's still available, they'll be able to provide you with the credentials or with the method to get them.