What sound source are you wanting to use? The sound from the drum kit, or a plugin drumkit(like EZDrummer). Here are some ideas(but I run PC so things may be a tad different for your Mac):
#1-using a drum plugin like EZDrummer; you insert the plugin on a stereo instrument track and put it into record-ready. Playing the E-kit should play the sounds of the plugin(which may need to be remapped so the snare pad plays the snare sound, the kick trigger plays the kick sound, etc). Most(if not all) drum plugins have presets for mapping to Roland and Yamaha E-kits, so they may also have a preset for Simmons.
#2-to use the sounds in the Simmons kit, you need to feed the audio outputs of the kit into your interface and select that input on a stereo instrument track(without any plugin). Alternately, you can use a midi track and a stereo aux track and feed the Simmons audio outs to this aux track(the instrument track combines the features of a midi track AND an aux track in one)
#3-go to youtube and search for some tutorials on setting up an E-kit with Pro Tools. It should work fine and I ran a Roland TD-10 kit with several different drum plugins with great success(until my biggest client insisted I get a real kit)
