The real problem here is that IBM doesn't put this documentation online, but the help files are installed along with the iSeries Client Tools. If you look around a bit, you'll end up with some directions like:
IBM System i Access for Windows > Programmer's Toolkit > Programmer's Toolkit
Which is technically supposed to open up a windows help file with the reference you're after. Unfortunately, Windows 7 can't open up .hlp files out of the box, so you have to download WinHlp32.exe:
So now you can open help files, but the Programmer's Toolkit still doesn't work - none of the links actually yield the intended behavior of opening up other .hlp files.
Finally, just give up and open the following file:
C:\Program Files (x86)\IBM\Client Access\MRI2924\cwbx.hlp
Navigating this thing is a pretty horrible experience, but the information you're after is in there. Ok, the information I was after was in there, but it's certainly a step in the right direction. Good luck!
And to answer the inevitable question "Where can I find cwbx.dll?" You can't download it, it's part of your installation media (so check w/ your sys admin).
While writeback has been around for a while, it didn't really get easy to do until Excel 2010 was released because wrapping a UI around it was cumbersome. Now that we have a simple writeback UI via the What-If analysis dialogs in Excel, it's pry worth poking around at how to get things to allocate the way you want.
Here's our starting point in the Adventure Works cube (note, I commented out all the scope statements in the MDX script that distribute quarterly quotas/targets to months - in AW, quotas only exist in the data warehouse at the calendar quarter level):