Investment accounts in Moneydance are used to aggregate and track the value of a set of securities, as well as calculate the balance that is created by all transactions in that account.

Investment Accounts can be created by selecting Account > New Account. Then select the "Investment" type, and enter the account details.
After you've created the Investment account, you'll need to set up securities within the account. This happens in three stages: creating the security, associating it with an investment account(s), and entering your holdings.