Please follow below steps:
1. Import prep data set from Admin tab
2. Clear cache
3. Open Header and change prep dataset to any other dataset and again try to select the actual dataset.
4. It will show you all columns including the new column you added in prep data set
5. Define column type i.e. array element, by variable or category. To avoid any confusion about column types, please take a screen shot of all columns and their types.
6. save it.
Hope this will be helpful. Feel free to comment in case of any problem.