There's a special area just for Truffles!  Slice the Truffles icon on the main menu (upper left).

In the Truffles play area, put your palm facing up and look at your hand to activate a popup for interacting with Truffles.  You can look and pinch or press on the stick, strawberry or whistle to use those items.

Throw the stick to play fetch, feed Truffles strawberries or call him over for a pat using the whistle.

To pick up the stick or strawberry from the ground, reach out with your hand until you see it highlight.  Then grab the air as if you were grabbing the item.  It will automatically move to your hand if done correctly.