In Rails 4, the parameter validation was removed from the Model, and now lives in the Controller. So, the "attr_accessible" line isn't used in the model anymore.
From the line you gave from the controller, there is a freq_params function in your controller, that should be like this:
# Never trust parameters from the scary internet, only allow the white list through.
def freq_params
params.require(:freq).permit(:description)
end
If it's not, change it to this, and remove the attr_accessible line from the Freq Model.