Click OK twice. 330 are the class midpoints. Experiment with other class widths and midpoints. Then be sure to close all histogram windows. You also can change the units on the vertical axis.
Percent is the most common choice since it is independent of sample size.
Minitab will construct a frequency polygon for your data. You have instructed Minitab to connect the midpoints of the histogram bars rather than draw the bars. You can adjust many features in Minitab by clicking first on that feature and then on the 6 next to the name of that feature at the top of the column. You may add a title to your histogram. Enter your title on the top row.