To designate an account as an Accounts Payable Account, it must be a Liability. To mark the account, go into Ledger & Payables, Accounts, pull up the Liability Account and mark the Accounts Payable option toward the bottom. Then, click Submit.
Note: If you have more than one Accounts Payable Account, you may want to select the one you use the most as your Default Accounts Payable Account.
Other Getting Started Help Files that may be of interest: