I haven't looked at Pizza Mario in a while, but if you're getting a login screen, you'll need to enter a username and password. Each Mendix app is created with an admin userid - default username is MxAdmin. You create the password yourself. To do this, look in the Project Explorer in the left hand side of the modeler. The first module listed is the Project - expand this. Double click security to open the security dialog box. Go to the Administrator tab to set your administrator password.
Hope that helps - Mike