Hello Geraldine,
If a function is not available in the report formula column, this means it is only available in calculated fields (i.e. custom fields). The problem with them is that one can't use date functions (e.g. Today(), Now()) in them.
Could you please clarify why you need to use the DateAdd() function? I think that the DateDiff() function returns a numeral, so you could just add or subtract another numeral from the result.
Thank you,
Roland