You probably don't need as much experience with xml as you might think. There's a section in the Adobe help that comes with the program that will help you with calculations. If you're adding up fields within the same subform, try this:
Sum(field1,field2,field3)
This is formcalc, and should be used in the calculate event of the field you want the sum to appear in. If you need to add up fields in separate subforms (like fields that span multiple pages) it's a little tougher. In that case, you need to specify the path to each field, like:
Sum((xfa[0].form[0].form1[0].subform2[0].sub2[0].L53),(xfa[0].form[0].form1[0].subform3[0].sub1[0].L60),
(xfa[0].form[0].form1[0].subform3[0].sub2[0].L71),(xfa[0].form[0].form1[0].subform3[0].sub3[0].L78),
(xfa[0].form[0].form1[0].subform4[0].sub1[0].L98))
Here, the name of my form is form1, the subforms are named subform2, subform3 and subform4, and the field names are L53, L60, L71, L78 and L98. Note the double parentheses after Sum and at the end of the script. This is also formcalc, and would go into the same calculate event of the field you want the sum to appear in.