SilverBullet Desktop is free for personal and education use; using it at work requires a Pro license. Tiers, prices and seats are explained on the pricing page and in the FAQ.
Open the Desktop dashboard, go to the Pro tab, paste your license key (it looks like SBPLS-XXXXX-XXXXX-XXXXX-XXXXX) and click Activate. A Pro license belongs to one user, who can activate it on as many of their own devices as they like.
To stop using a key on a device, click Remove key on the same tab.
Manage your subscription, license keys and invoices from your account page. The FAQ explains how to get an invoice with your company details.