She's probably just a little nervous. All our cats we found outside when they were kittens so they were use to going outside. But, there is something called a kitty-door, like a doggy-door but smaller. If you show it to her, she'll gradually start to use it. It works even better when you feed them out on the other side of the door.
But, just remember, she's young and everything is big and unfamiliar to her. Roll around on the grass with her a bit. She'll get use to being outside soon enough.
Some cats just don't like going outside. Agoraphobic. If she doesn't want to go outside, I wouldn't force her to - she'll be happy enough staying indoors if she's that kind of cat.
Try letting her out just in the day at first. You may also want to go out with her or hold her so she feels more secure. Eventually she should be happy to go outside unless she just isn't an outdoor cat?
It took my kittens months before they were happy to go outside, i just let them out for short periods of times and would stay with them, and pick them up. Then if you just leave the door open they will gradually start going out by themselves. I think its quite normal as they are getting used to their surroundings.