OpenERP configures a basic accounting structure when you install base modules, such as Sales and Purchasing. To access all of the accounting configuration options, you must install the Accounting and Finance module. If this module is not already installed in your configuration, navigate to Settings and click on Apps to pull up the available applications. Look for Accounting and Finance and click on the Install button to install the module.
After you have installed the Accounting and Finance application, your menu structure at the top of OpenERP will change. Before installing the module, you likely had an Invoicing menu that contained the necessary options for the Sales and Purchasing applications. Once the Accounting and Finance module is installed, the Invoicing menu is replaced with an Accounting menu and is populated with several more options.