Click on the dropdown to see the list of datasets to plot.
If you are a subscribing customer then you could choose one from under 'Your Company Datasets' instead.
You will see the name of your dataset and the level of data it contains.
If there are multiple columns/fields in your dataset then you will need to select which one to plot by clicking on it.
If you click on the wrong one just click on the right one after it and the selected field will change.
If there is just one field then it will be selected for you and this step will be skipped.