Created a forester but...

[Squidbat]

... the guys have the saplings but wont plant them. How do I make them plant the trees?

[Schalimah]

The Forester only re-plants cut trees.
If there never were any trees around, they won't plant any.
You may also need someone with farming priority to plant the trees.
Making saplings runs under crafting if I am not mistaken.