Ah, you have a slight typo (not sure if that’s contributed to the confusion) - it’s のだ.
Whole thing:
山形さんが打ったゴルフのボールをひろってくるように、おしえたのだ。
It’s referring back to the previous sentence and explaining what it was that he trained Tobotobo to do.
Yamagata-san doesn’t pick up the ball 
it starts off with a relative clause describing the golf ball(s) - the object of the sentence is “golf balls which Yamagata-san had hit”.
You’re right that it’s おしえる - it’s in the past tense. The の turns it into a noun, and then we have the copula だ.
“[the training mentioned in the previous sentence] was teaching the dog to fetch up golf balls Yamagata-san had hit”
Does that make sense?
Also, feel free to ask questions in these threads! Lots of us have them set to watching so we’ll get notified, and at the very least I’ll be notified because it’s my thread 
Woah, just noticed that was your first post in the forums! Welcome! 
By the way, you can edit your posts, like if you forget the page number initially 