I want to enter the character
in Excel v.X. So I guess the proper question is, "How
do I enter the flat character in Word or Excel?"
As mentioned earlier, you must use a modern Unicode app, NOT WordX (or ExcelX either). The answer to your question is you need to upgrade to Excel2004. (There's a free trial available of Office2004).
And
why does the tool that's available in OSX not help me
do it?
Because you are using the wrong app. OS X is a Unicode system. MS OfficeX is just a slightly modified OS 9 app and cannot use a huge amount of OS X's features.
If you must use OfficeX, the only solution is to download a non-Unicode, OS 9 type music symbols font and set the app to that and see if you can enter what you need from the keyboard or via the Character Palette in glyph mode.