For selecting a partner who is already in your tree:
1) Select the person to whom you wish to add the partner
2) Click on the "Add partner/ex" button
3) Click on the "Partner with person already on tree"
4) A new dialog will appear with a pulldown list at the top; after some time (seconds) names will appear. Start typing the first name quickly, followed by the last name, but don't pause between key strikes. Names that match what you've typed should appear (with birth/death dates if available).
5) When you find the person you want, highlight it and click the "Add as current partner" or "Add as ex-partner".
Unfortunately, there isn't a way to add a parent or child in this way.