I get the feeling from your Opening Post that you are not really familiar with what the Theory of Evolution says. So maybe it would help if we started with some of the basics.
I have searched high and low to find any evidence supporting a beneficial mutation. All I have ever seen is mutations where it seems as though it would hinder the creature, not help it.
Whether something is beneficial or not depends on the environment the critter (plant type critter or animal type critter) is in. What is beneficial in one environment may well hinder the critter in a different environment.
Let's look at some basics. If the critter lives in a snow covered area, then a mutation that makes the critter whiter is beneficial. BUT... if the weather warms up and the snow melts, the critter that is brown will have the beneficial mutation.
The Fact of Evolution only involves Change over Time and we can see from the records that critters have changed over time.
Also, I have never seen any evidence of a creature gaining anything new, which be required for the evolution theory.
No, that really isn't quite true. There is nothing in the Theory of Evolution that says critters have to gain anything new. It does though explain how they can gain new things.
This really makes me question evolution, since it's based completely off mutations.
Again, not quite right. Mutations (and there are many different types of mutations) are just part of the Theory of Evolution. Another part is mentioned above, and that is Selection. Natural Selection is the environment that the critter lives in. Some critters (and remember we are talking about plants too as critters) will be better suited to a particular environment then others. Those critters will be more successful reproducing and so their genes will over time become the population.
Anyone so limited that they can only spell a word one way is severely handicapped!