its Unknown

Then in the opening movie why is she sitting with the good guys toward the end when Raiden is talking?

she is with sub-zero.(his student)

it's hard to say, since certain elements of the game show contradictions in her standings. she's shown sitting with the good guys when rayden is talking ot them, and she is also supposedly a new member of the lin kuei, which is now good. but in her ending, it has her betraying sub zero and trying to take the dragon medallion. so it's unclear to me, although it should be noted that all the endings can't be true, since they do obviously contradict one another and it will not be known until mk6 which of the endings are true and which ones were just made up so the character could have a cool ending.

Since she betrays Sub-Zero, her Sifu and Grandmaster, I ́d say she ́s evil. But I think she might yet be rehabilitated (if she survived her ending, as Sub-Zero ́s ending seems to imply).

i'd say she's good, although she wanted to be grandmaster herself, she did come with subzero to stop the dealy alliance.
They could make her evil in the next mk, where her storyline would something along the lines of:

"A loyal servant to Quan Chi, Frost was sent to the lin kuei to spy on Subzero and hopefully to overthrow him and rule side by side with the red dragon leader Mavado."

She does have an unknown past so it could lead this way or any other way, you know what i mean.

if you have beaten it with er you would realize she is good until she goes to take the dragon amulet from subzero to become grand master wich is wh yshe then turns bad so you see she is both wich is why they put her as unknown because she is unreliable and changes quikly

its obvious that she is good because she is with the lin kuei and they are good.

If I have to put my money on something, I'll put it on her being evil. I mean, that would just be a little too logic. The student that turns rogue. They are obviously not making Frost's ending come true, since we still don't know anything about her, but she is definetely going to have the bad intentions all the way, and when she can use the Dragon Medallion, they will have her stealing it for sure. For now, she's still too unprepared to handle either the Medallion or the intricated Storyline. But when she's ready, write my words: She will be the Storyline!

Earthrealm_invader Wrote: its obvious that she is good because she is with the lin kuei and they are good.

Not necessarily: there can always be bad elements in good organizations. If there ́s one thing I hate, it ́s a cop gone bad.

Treason ́s a pretty evil thing in my book, and Frost has proven to have treacherous intentions. But like I said, she might yet see the error of her ways... or not. She could also grow to become a thorn in Sub-Zero ́s side, and make him wish he ́d let her freeze to death when she tried to steal the medallion from him (if her ending ́s given canon status and linked to his, that is). I suppose only time will tell.