I just purchased a MES 40 a few days back so my help may be somewhat limited.
Make sure you season the unit first. Mine said to heat to 275 for 3 hours. In the last 45 minutes to add wood chips for smoke.
I used water with my first smoking. I did a rack of baby backs and some drumsticks.
I used the water tray because I figured, what the heck, they give me a water pan why not use it. I line it with foil to make it easier to clean.
When I did the first cook I start the smoke about 30 minutes before I put the meat on to get the racks hot. I also used the hottest water I could to place in the water tray. This way it would come up to temp quicker.
THE BIGGEST LESSON I LEARNED:
When I got ready to remove the meat, I opened the door and the steam and smoke soot had collected on the door. When I swung the door open it ran down and poured out onto my pavers on the lanai. That was something I didn't suspect and was pretty upset about when it happened.
Sorry I can't help you with the rub but others will chime in.
Good luck and happy smoking