DHC-6 Twin Otter - Bendix/King KLN 90B GPS Navigation System

I am looking for some guidance. I've read and re-read the Aerosoft manual in an attempt to understand the functionality of this device. Failing a grasp of the functions from reading, I embarked on a flight in which I could experiment to test my knowledge and fluency in its operations. I have also referred to the actual manual for this equipment, but fear that many of the functions are not featured in te sim version. I am using the KLN90B Navigation System in the Aerosoft Twin Otter. From within FSX, I created and applied a low airway flighplan which contained numerous VORs and intersection points. In the most part, left untouched, I likely would have gotten from point A to point B no problem flying by hand using the KLN as a source of reference only. I'm not certian how to use this in the capacity of an "auto pilot". In my monkeying around, I somehow added an approach routine to the end of my previously defined flight plan in which I was unaware I had done. Since I was flying by hand, it likely would't have had any impact on my flight as I would have just ignored the reference. However, as my previous flight plan failed to include a valuable waypoint at the end of my flight (it was unavailable in FSX to be included during planning), my thoughts had me think that if I used direct to, and entered the missing waypoint, I would get there. (My ignorance on the function of direct-to). For the life of me, I could not enter any characters in the 5 spots highlighted by the blinking cursour. In this action, somehow my aircraft took over! I looked at the flight plan page, and on top it was flashing "ENROUTE". I scrolled down, and my guess was, I was flying "direct-to" that inadvertently added approach that was added to my flightplan 1 hour earlier in my flight ... which BTW was NO WHERE near where I should be going. Not only that, but I was on a steady descent from 12,000'. I had no control over my aircraft. Flipping the NAV/GPS button still kept me in autopilot, but the bearing changed as I changed the OBS of the HSI. Altitude continued to descend. So ... this experience has left me wondering, and very confused ... a) If I need to ammend my flight plan by adding points midway, or at the end, that can't be added through FSX, how do I do that? I needed to go to the first point of my destination's STAR, which was not in either FSX or the KLN database.:( How do I delete a point, in this case an approach, from my flightplan?c) How did I unknowingly turn on the "autopilot" that was taking me to the approach waypoint at the end of my flightplan. Direct-to???d) And if I inadvertently commenced a direct-to (auto-pilot), how do I disabe it to resume manual navigation? Obviously in this case, the aircraft was flying itself. I needed to disable it to resume.e) What's with the altitude? Why did it assume, where did it get the instruction to descent ... I don't ever recall defining any altitudes?f) How do I use this as an "autopilot" (if it's supposed to perform like one) when I have a flight plan established? I had a hard time keeping a steady altitude. An autopoliot would have helped immensely. Thank you in advance for your patience. I feel this navigational system is quite advanced, but has a steep learning curve and unforgiving to those without a strong grasp of it's operation.