Hello,
In version 6, you will find "Joomla! integration" in the AcyMailing configuration, tab Subscription. There you can choose to create a user on site account creation.
Do you have the latest version installed? (currently 6.1.8)
We will add some more option in the next release by the way.
In version 5, you can find the plugin in the AcyMailing configuration, tab plugins ("AcyMailing : (auto)Subscribe during Joomla registration"). In the column Integrations with AcyMailing. You can also find it in the Joomla plugins section.
This plugin is installed with the component.