The slap option that is accessed by pressing the "m" key 3 times in Notion 6, will not affect the Double Bass. If you are looking for a jazz style bass, I might suggest going into "Guitars/Basses" in the Score Setup window and selecting the "Upright Bass." This will give you a "walking" bass that is plucked. If you get a chance, load up an electric bass, add some notes, press the "m" key 3 times, and add "slap" to your score. You can drag the "slap" function out across a passage to hear it slapped.