In Field Layout in the Designer, the right sidebar contains a Field Tools tab. In this tab, you will find the different type of fields that you can add to your template.
To Add a Field, click on one of the field types and drag it over to the PDF view and drop the field.
Once you have the field placed on the PDF, you will then want to resize it to match the field size of the PDF.
After the field is in place, all field types can be updated to enter a Field Name, Display Name, and determine if the field is Mandatory.
For more details about other field level settings, click on the links by each field type: