Make sure you select the Multiple Select and Relational Storage checkboxes. Multiple Select allows you to select and store multiple values from the list, while Relational Storage makes it easier to integrate that property into other queries by storing each selected item in its own entry in the database (without it all the selected values are stored in one row, concatenated into a VARCHAR column), and to store more than a few selected values (since the column allows at most 255 characters in total).