The DataValueField and DataTextField are both just looking for string representation of what they're supposed to use. You would set your DataSource to some sort of IEnumerable that actually has those properties, so assuming db.categoria has the properties "codcategoria" and "dsccategoria" you could do something like:
testesiteEntities db = new testesiteEntities();//create object
//select object
rblCategoria.DataValueField = "codcategoria";
rblCategoria.DataTextField = "dsccategoria";
rblCategoria.DataSource = db.categoria.Select().ToList();
rblCategoria.DataBind();//define valores no bullet list