The easiest way to interface JAMin with Ardour is to use an insert on the Master channel of the mixer. The figure below shows an screenshot of an Ardour mixer. There is an insert (in red) on the on the output of the master channel. This can be done by right-clicking in the block and selecting new insert.
Once the insert appears, double-click on it to see its details like the figure below. If JAMin is already open, it will appear as available connections for both the input and output. Set out1 to jamin:in_L and out2 to jamin:in_R. This is done by dragging the JAMin labels into the relevant block. Similarly, set in1 and in2 to jamin:out_L and jamin:out_R respectively.
Your Ardour track should now be playing through JAMin. There is one more thing to do - go to the connect tab in JACK and disconnect JAMin out_L and out_R from playback_1 and playback_2 respectively. If you don´t do this, your track will playback from both Ardour and JAMin.