Initial Balance

You can set the 'Initial Balance' (sometimes referred to as a starting or opening balance) when creating an account. This can also can be altered at any time.

Open the register of your account by clicking on it in the left sidebar, and then navigate to Account --> Edit Account from the Moneydance menu bar.

In the window that appears, one of the fields is for the Initial Balance of the account.
This should be set as the balance of your account before the earliest transaction within Moneydance.

Moneydance can also calculate your Initial Balance.
For example if your current checking account balance is $200, and the balance shown in Moneydance is $27.65, you can enter 200-27.65 (use the minus sign) in the Initial Balance field and hit enter.
Moneydance will automatically calculate the proper Initial Balance.