When you first create a new project, you must select a base dataset.
To select your Base dataset click on the "dataset (click to select)" button, which takes you to the Data Library. Click the green "Select" button on the left side of the dataset you wish to use.
Once you have selected a dataset, you are taken back to the Paxata Project; this becomes the start step of your Paxata Project . Once you hit the save button, you can work on the selected dataset on your Paxata Project.