Unfortunately the CodeIgniter Model doesn't have access to the values that you're looking for. But there is hope! There are a few ways to go about having getting access to the parameters that are loaded from the request, choose your poison, but having a defined contract interface for your model is, in my opinion, something that makes writing code easier and less of a hassle to maintain down the line.
A reference to the CodeIgniter object can be grabbed like so:
$ci =& get_instance();
$session_id = $ci->load->get_var('current_id');
Of course this would be the nasty way to go about getting at your data. A better approach would be to pass it in to the model itself as a parameter. You're not going to be paying for much in doing so, and it gives you a contract signature on the method where you can point and say "Hey, this should be the current_id of the model:
function sets($session_id)
{
...
}
And, of course, you would just pass in the value you wish $current_id to be as an argument in that function call. This gives you the flexibility of not having to rely on the fact that you're populating the model from a POST HTTP request.