Basically, you have mis-named your Privileges class. If you read through the docs, you'll see that the model should always be named singularly, ie: Privilege.
However, there is a solution that will work most of the time: Hard code your model name into Privileges like this:
Code:
class Privileges extends DataMapper {
var $model = 'privileges';
...
I recommend changing the name of the class, if you can, because there are a few other places in DataMapper that might eventually give you trouble down the road.