an optional and cutomizable element

[Angelica Perduta]

I'm writing my first gstreamer application at the moment and I got
stuck already.
Please can someone steer me in the right direction:

I want to add an optional element in my manually constructed pipe.
So I thought of having a configurable string to pass to
gst_parse_launch (this will then implement my customizable filter).
The problem is that I can't understand how to link to the source and
sink pads of the element that  gst_parse_launch creates.
So simplified down it's like this:

[code]
GstElement *pSink = gst_parse_launch("audioconvert ! autoaudiosink", &pErr);
GstElement *pSource = gst_element_factory_make ("audiotestsrc", "source");
pPipe = gst_pipeline_new("test_pipe");
gst_bin_add_many(GST_BIN(pPipe), pSource, pSink, NULL);
if (!gst_element_link (pSource, pSink)) cs::fail("failed link\n"); //
it fails to link them :(
[/code]