I've been trying to browse info about this for a while now. The user defined functions in Excel 2007 just make no sense to me whatsoever.
According to all the tutorials, it should be really simple. Just press alt + f11, insert a new module, and enter the VBA code there. At this point typing '=FUNCTIONNAME(parameters)' into any cell should make it work.
It did, at first. Then upon closing and reopening the worksheet it stopped working. I checked to see that macros were enabled, but either way, that didn't help. Then I browsed the graphic UI from the function button to locate user defined functions. I found it there by name of 'MyExcelWorksheet.xls!FUNCTIONNAME.FUNCTIONNAME' and clicking that, it started working again.
Now, a day later, I get back to working on this and it's broken, again. I just get #NAME? error or some "Function Arguments - Function doesn't use any arguments" -popup (translated from Finnish Excel, not sure what that msg is in English) when I select the function from the GUI.
So what does it take to just create a function, and actually make it work and KEEP working? For the life of me I can't figure out why this has to be so difficult. :) I'm starting to think my company has some weird settings enabled or whatever, because judging by the lack of information I've found on the subject, this isn't supposed to happen.