Here are some examples where you enter the data directly into the bar:
exponential fit of {1,2,4,7,17}
http://www.wolframalpha.com/input/?i=exponential%20fit%20of%20%7B1%2C2%2C4%2C7%2C17%7D
exponential fit of {{0,1},{2,2},{4,4},{6,7},{8,17}}
http://www.wolframalpha.com/input/?i=exponential%20fit%20of%20%7B1%2C2%2C4%2C7%2C17%7D
There are no examples showing how this can be done with a dataset. I am not aware of any. It's very possible that the Wolfram|Alpha doesn't recognize that query yet for datasets. If a query doesn't work that you expect to, you can contact the Wolfram|Alpha team and leave them a suggestion at the bottom of the results page. This helps them know where to focus the future developement of Wolfram|Alpha.