Here’s the complete route.

If you can’t find your marble, it is most likely on the lowest level. To check it, shift all sliders up and hold the box vertically as shown below. Tilt it right and left – you will see the marble in the holes. To release the marble, turn the box upside down and tilt it

Here’s the full guide through this puzzle. Shift the right slider to the top and bring the marble into the slot. Shift the slider all the way down. The marble should fall down below the 2nd cross. Bring the marble to the 3rd cross and shift the right slider as shown below. Turn over the

There is a pusher near the locked marble

You have to release one marble using the other one.

Bring the free marble between the locked one and the pusher, then use the pusher.

You need to use both marbles in the next part.

Bring both marbles as shown below, then turn the mechanism clockwise. The last puzzle will be unlocked.

There is an element with an arrow sign.

Remove the key.

Pay attention to the arrows showing rotation.

The outer arrows show how to rotate the box, the inner arrows show the rotation of the central part.