How to add an existing field to a fillable form

I am working with a fillable form in Adobe Acrobat and a few text fields and a radio button field are in my list of fields (in the right window) but not on my form.  I cannot figure out how to add these fields which have already been defined onto my form.  I cannot drag and drop them from the field list.  When I click Text Box or Radio Button it creates a new field.  How do I put a field that has been defined onto my fillable form?