To add Subscription form you should follow below steps.

Step 1: Click on Manage Websites

Step 2: Click on settings icon of website which you want to modify

Step 3: Click inside General Tab in settings panal

Step 4: Inside "Subscribe HTML Form Code" Text Area add your autoresponder code. and click save.

All the changes made by you will get reflected on website. if you face any issue please let us know.