Ok so you probably set it up properly, and now you want to add people to go onto the family tree.
If you add a category called "Family".
Then add a new post by clicking on "Add post".
You can make the title of the new post the name of the person, and at the bottom of the page you will be able to set thing like birth date, gender, etc.
In the dropdown you can select who is the mother and father of that person. The dropdown will only show people that are alreade created as posts and are included in the Family category.
If you try this then it should work.
If it doesn't then let me know. Some people have reported a possible conflict that you can get with other plugins so if you have done everything above, then you might have a problem with another plugin.
Let me know how you get on...