Why wont my form add in the calculation?

I am trying to make sure that my fillable form will sum two fields together. When I go into properties and click the Calculate area and click on the fields I want summed, they wont pop up in the dialog box. In fact when I try to write in the field names, there is a bell that tells me it can't be done. What should I do? Is there something I need to check on my form to make this property work?