You need FileMaker Pro Advanced to work with custom functions.
Adding a cf can be done easily by copy&paste (function name, parameters, function)
in short / 'pseudo code':
- Add New function
- copy name from briandunning, paste it into the name-field of the cf
- copy parameter (one by one), paste it into the param' field, klick the '+' to add it
- copy the formula an paste it into the big field
if there are errors when You leave the cf definition box, there might be problems with parameters - or the FileMaker version..
After creating that cf, You can use it just the way You use standard FileMaker functions as shown in the example on the briandunning site
If You don't have an Advanced version, You can create a formula using a couple of helper-fields or a script that fills/calculates $-variables and calculates the final field. All methods will need some basic knowledge of FileMaker - but that's not rocket-science (-: