Why are omnivores found at different levels of a food pyramid

I don't think you mean food 'pyramid' you mean food chain or web right?
Well then, omnivores are found on different areas of the food chain because of their trophic level.
The definition of omnivores is this: An animal or person that eats food of both plant and animal origin. Therefore, their trophic level placement would depend on the other organisms included in the chain.
They are found at different levels because there may be another omnivore who eats only plant origin, this organism would be found towards the bottom, whereas, the organism that eats mostly animals would be situated closer if not on the top of the food chain.
For example, If we were comparing pigs to humans, humans would be the first order consumer as we eat the pig. The pig would still, however be considered an omnivore, but its placement on the food chain would be a lot lower than ours.
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I hope this clarifies your understanding!!! =)

Because omnivores eat more than one thing, and eat at more than one trophic level.

Consider the meal of salad, beef, and salmon before you.
When you eat the salad, you're a primary consumer.
When you eat the beef, you're a secondary consumer.
When you eat the salmon, you're a quaternary or even higher consumer (salmon are predatory fish).

Depending on their role in an environment, omnivores can be food for higher predators, or may even be the top predator themselves. Example: mice, which are omnivorous, fall prey to snakes, which in turn can be preyed upon by bears.

they eat both animal and plant material