In adding a subscription form, you will have to connect it to an autoresponder.
Edit the page where you would like to add a subscription form. Go to Main Modules and drag Subscription Form then drop it to your page.
In the pop-up form, choose the service you would like to use for your autoresponder.
For example if you choose GetResponse, go to your GetResponse account then copy your API Key and paste it in the field provided for API Key and your account name. Once you the fields filed out click the Connect button. Continue to customize your subscription form. Once done click the SAVE button.
Here's the example of a subscription form created.