In order to use the Visualforce pages associated with custom FinancialForce objects, you must ensure that all Visualforce pages are enabled on all relevant profiles.
From Setup, click Manage Users | Profiles.
Click the name of the Standard User profile.
Scroll to the Enabled Visualforce Page Access section.
Click Edit.
Enable access to all FinancialForce AccountingVisualforce pages. All FinancialForce AccountingVisualforce pages in the core package have a c2g prefix.