If you have used oil in FuelPHP 1.5.1, you may have noticed that the controllers generated by the oil scaffolding do not use the configurer controller prefix. What this means is that if you are using namespaces or something other than the default controller prefix, the scaffolding controllers will not work.
The good news, however, is that Harro (WanWizard) commited a fix to the oil git repo last week so you will see a fix to the issue soon, perhaps in the FuelPHP 1.6 or 2.0 release. Until then, try to stick to the default prefix (fuel/app/classes/controller directory and no namespaces) or you will have to refactor the code generated by oil to get it to work with the rest of your application.
While you are learning FuelPHP, consider getting a copy of the FuelPHP Guide from the Kindle Amazon eBook store.