When you open a clip, it selects it as a source.
If you open it from the playlist, it's addded as a new line in playslist (usefull to take it again if you need it more that once).
That's the first rule. Then :
Select the track you want to add it to (I usually do it before and place the cursor if I know where I want it).
Then I open the clip (or select it from the playslist). When you're sure you selected it, you can move it to the timeline :
Use the little icons just above the tracks and selec the one you need (to add at the end of the track, insert at the cursor, paste above starting at the cursor...)