The first time, choose Sign up and enter your first name, last name and email; from then on choose Sign in and enter only your email.
Type the code on the Enter the code screen and click Verify code. The code is valid for 10 minutes, and the screen counts down the time the SyncNow server gives it. If the code is wrong the screen says how many attempts are left; a code already used for a sign-in says This code was already used, and one replaced by a newer email says so. Resend code sends a new code 30 seconds after the last code was sent (until then the link shows the wait). An expired or already used code never blocks a new one. If you go Back and then Continue while your code is still valid, SyncNow sends no new email and goes back to the Enter the code screen. Each code email has the time it was sent in its subject. On the Sign in screen, Stay signed in keeps that PC signed in for much longer.
Once signed in, the app shows the projects you are a member of, read from the SyncNow server, so a new PC shows the same list without copying anything. Account, in the sidebar, shows your other active sessions and has Logout. Then add your first project: see How do I add my first project?.