hello, i hope i can help you a little bit with your first problem. In the MCDU at the first init page you fill in the Departure and there after your Arrival so like this (egss/egll) according to your flight plan made in sim letter which already states what your departure runway is and your arrival runway is stated. If everything is correct you will see a following page with waypoints of your simbrief FMS file. With the arrows you can go up and down to select the sid and star and don't forget to click insert in all cases otherwise it will indeed not work.
Good luck.