You can activate your SproutVideo account at any point during the free trial period. To do so, click on the link that says "Activate your account now" near the navigation bar towards the top righthand corner of the page:
You can also select "Activate Account" from the "Account" drop-down menu, or from the Subscriptions tab:
If it slipped your mind, and your free trial expired, not to worry, you can still activate your account within 21 days of your trial expiring without losing your work.
You will have to enter your credit card information, and once you activate your account, you will be charged for the first month of service. Pick your plan, fill out your payment details, click "Activate Your Account!" and you're done!
After filling out this short form, you will be taken to your videos page, where you can start uploading videos and customizing your account.