This article describes how to add subscribers one by one. How to import multiple subscribers is shown here.
This will be fast and painless, don't sweat it :-)
First, just click on the Subscribers section on the left side of the menu.
Click on Add new in the upper right corner just below your profile.
You can add the new subscriber into one of your audience Lists or create a new List for them.
Then fill in the subscriber's details and decide if you'd like to send them an opt-in email. If you send an opt-in email to a subscriber, the subscriber's status is Unconfirmed and you cannot send any campaigns to them, until they confirm the opt-in email and their status is updated to Active.
When you are done, click on Create subscriber.
New subscriber has been added. Now click on Awesome...
... and check your new subscriber in the Subscribers section.
Your new subscriber is now ready to receive your campaigns!