Translation:The movie is interesting.
It is ok to use 가 instead of 는. Both are correct grammatically. Korean people use 는 to establish the topic of the conversation. Here for example the topic is now 'movie' . Once established they use 가 to add information about the topic. If in doubt use 가. It will be totally ok. For more understanding you may refer "learn Korean with Billy" on YouTube. He has a 2 hr explanation on topic marker , subject marker and object markers. It is highly helpful.
They kinda are the "the", you don't say "movie is fun", you put a "the" in the beginning
I agree with your complaint. English is not my first language and correctly using the articles is sometimes a challenge. I am taking Korean course in order to learn Korean, not English. The course has no problem with singular / plural ambiguity, but sometimes it says I am missing a word if I skip "The" at the beginning of the sentence. While there was no "the" in the Korean sentence :-(
[Isn't "있습니다" mean "There is"?]
It means more than just that. It can mean "exists", "is", and "has", for a few examples.
And if you mean that you think the sentence should use "입니다 (이다)", it would not make sense. That verb is used for describing a thing by equating it with another thing. You can't use it with an idea like fun/interest.
The adjective 재미있다 can mean either "to be interesting" or "to be fun." Both answers should be acceptable ("the movie is interesting" or "the movie is fun")