Go to your Cockpit, scroll to Slaves/Followers/Masters -> Accounts.
Click under Plan column and select a Subscription or Prepay Plan for your account. To use your Free Trial Credit, you have to assign the Prepay Plan.

You can also do it in the Account Settings, just click on the Account column.

Important to note that a Subscription Plan or Prepay Plan has to be assigned to you trading account in order to have access to all features, settings and unlimited trading.