If you used the bank import during the account setup, the starting balance should have been automatically created for you. If you did a manual setup, there was a box at the top of the setup wizard for the starting balance.
In either case, if you missed that and already have entries in your calendar – don’t start over just for that. The easiest thing to do is make a correction entry on today’s date (but this should be just a 1 time entry – you should not make regular corrections). Take the balance that CalendarBudget says today has and subtract it from your actual account balance today, then create an entry for that amount. That will make today’s balance correct.
Normally we do this same thing, but at the beginning of your data, but that may not be possible now.
If you haven’t made many entries since you started – you can always reset your account (Setup -> Account Setup -> Reset) and then use the Account Setup wizard to import your account details and it will automatically find your correct starting balance.
If you’re still stuck – let me know.