How can I start the drums 10-15 seconds after the song starts

You can program the n-Track Drums in two ways:

you can use the step-sequencer built into n-Track Drums to program the drum sounds used by each measure of the song. You can open the step sequencer clicking in the "Steps" button in the n-Track Drums property page. You can then assign a blank (i.e. no sound) pattern to the measures corresponding to the first 10-15 seconds of the song and then non-silent patterns to subsequent measures.
alternatively you can use the n-Track Drums as only a MIDI synth, create a regular n-Track MIDI track, program the drum notes during the length of the song by editing the MIDI track in the n-Track Piano Roll window, and assign the output of the MIDI track to be sent to the n-Track Drums plugin with the 'Output' drop-down list in the MIDI track properties box that appears double clicking on the MIDI track.