When do you use "נמצא" and when is it optional?
It is always optional. It translates roughly as "is located". If you are learning German you will find similar optional verbs of location such as "befinden sich".
Can זבל be used instead of אשפה ?
אני חושבת שכן אפשר...
Why is it not בתוך?
Well, בתוך means "inside". That is possible, of course, but not as a translation of this sentence.
The garbage is inside the garbage bin. האשפה נמצאת בתוך פח האשפה.