You're normally signed in already by the time you open the panel, because you signed in inside the Butter Motion app before it installed the panel. This page covers the panel's own sign-in, for when you need it again: your session expired, you signed out, or another device took over your one connected seat. It works the same way as in the app: the panel opens a page on the website, you confirm the device there, and the panel connects to your account.
You're in. Head to Your first generation.
If there's no account for your email, the site says Account {email} not found. and offers Create a free account. It opens the Free plan sign-up for that email: no card, 50 credits every month. Create the account, then sign in with the code from your email and connect the device as above. The panel's sign-in menu has the same Create a free account button. See Plans: Free, Pro & Studio.
Once connected, the panel stays signed in for 30 days before you need to sign in again.
Your plan works on one machine at a time. Connecting the panel on a new computer signs out the previously connected one, and the old device stops working right away.
This is normal and expected. If you move between two computers, just sign in again on the one you're using. You don't lose anything. To see or manage your connected device, see Devices.