Problems with calculation tab, in properties, text fields

I'm creating a fillable PDF sales order, which calculates the figures for me.  after setting up the form, i went to the properties in the text field, and tried to set up the calcuate section.  When i try to use either the sum or product, it takes me to the pick area, but it won't let me choose any fields.  They get highlighted in blue, but I can't get the check mark in the fields i need to select.

I've created a form using the calculate field a while back, on a different computer, and when i went to see if i could edit it, i couldn't.  I could not select more items or deselect the ones that were set up.  I recently switched to a mac, and am running parallel with which i am using my adobe software.  not sure if that makes a difference or not?

I don't create that many forms, so hoping their is a simple solution to this.  My software is older, it is Adobe Acrobat XI Standard...

thanks in advance..

Kimmie