I have found that (counter-intuitively) clips load in reverse if you select them all and then use the "add to slideshow" down arrow, but if you drag and drop them into their position directly, they'll add in the correct order. To add individual clips, drag and drop the clip to the position where you want to insert it. You will see a small red arrow at the top to the LEFT of the already present clip, which indicates that the new clip will precede the one that is there already. Experiment by moving your new clip from side to side as you hover over the clip series. You should see the small red down arrow adjust its position as you do so, always indicating where your new clip will appear in the show.
Hope this helps!
