The formula tool is pretty straight forward.
To add you can use something like this
Create a new formula, give it a name,
then in the place provided..
type something like this. You can also drag and drop report fields.
({field1}+f{field2}+{field3}) and so on...
another form may be
numbervar x;
x := ({field1}+f{field2}+{field3});
x;
The last form uses a variable to hold the result of the addition and then returns the result (x) to be displayed. A couple of things to keep in mind in using forumulas the ':=' assigns a value while a straight '=' is a comparison. This is useful in creating more complicatied calculations. Also the semi colon ';' if not properly placed can create a little havoc. It represent the end of action. In the second example is does the addition then says stop. Anything that follows is not related to the sum amount being assigned to 'x'
Brian has chapter of his book available here and believe there is one on formulas. If this is correct give it a look because it will cover this matter in far more detail and even better it has pictures.
Regards,
John W.