To sign up for the Einstein Vision account for free, navigate to the https://metamind.io/.
Einstein Vision was originally metamind.io. With the Salesforce acquisition, it is now a part of Salesforce Einstein.
Click on the free sign up link at the top-right corner--you will need a Salesforce account to sign up to obtain a Private Key:
Click on the download key to download the Private Key to your local machine. Please keep the key handy as we will use it to sign the JSON Web Token (JWT) and obtain the access token to access the Vision Services API. Note that this Private Key can be used to call the Einstein Vision Services from any system. For the Heroku add-on, you will notice that the add-on automatically provisions the Private Key, and we can use that in our code for authentication.