Your API keys to use TrustistEcommerce services can be found in the TrustistTransfer app.
To access them, log into the app, tap on the settings cog icon on the right hand side of the lower nav menu, then scroll down until the following section is visible:

Keys can be regenerated by clicking the Regenerate Keys button, this can be used if, for example, you believe someone may have gained access to them somehow and you wish to secure your account against unauthorised access. This is analagous to changing your password in a username/password login scenario and will result in the previous keys being immediately invalid, so only press the button if you are ready to update your ecommerce platform.