After adding a Subscription question to your form, click the cogwheel question to the right to configure it. You can select plans from your connected Stripe or Braintree account under "Available Plans" to add them to the form, or click the current options under "Selected Plans" to remove them.
Plans are pulled from the pricing plans set up on Stripe or Braintree directly. If you're working with Stripe, the mode (live or test) you choose on your form must match the mode you created the pricing plan in on Stripe or the plan will not appear for selection.
Subscription plan prices will not show in the form or checkout total price. Instead, the subscription plan price will be referred to in the message below the checkout total.
Subscription plans are loaded from Stripe or Braintree when you add them to the form. If you change the subscription in Stripe afterwards (e.g. add a trial period) you will need to remove the subscription from the form and add it again. This will load the new subscription details and any new submissions will include the trial period.