I created a formula field with Now() on my Account object but it doesn't show up in the process builder as a selectable field. A test blank Text field does show up, so is there are problem with using date fields in process builder? How were people able to get this workaround to work?
Try changing your formula result to Date vs Date/Time and it should allow you to pick the field. I tested this out in my Dev org and noticed the Task Due Date is Date Only. So you may need to have your formula result in Date only.