To add up totals based on the results of elements in your form, you need the Total Field.  Please note that as this is based on previous questions in the form, add those first and then options are automatically given to you as part of the sum.  (If you want to do more than just add things up, you need the Calculation Field).

Eventree will look at any numerical, decimal or number field added to your form and display all options automatically in a list (see below).  It will also include any Checkbox Group, Radio Group, or Dropdown List elements that have costs associated with them. Simply click on the options you want to select (use shift or Ctrl button to select multiple options) and they will go from blue to grey when you close the element:

For more complex calculations, like adding VAT to a total, or multiplying a quantity of values, see 'Calculating a total value' help