For some reason Moneydance looses the human determined File Set Name and uses the underlying ID in its place. This is something the developers will have to resolve.
However, as I understand it, on the third machine you can rename the data set to match the other systems (while Moneydance is shut down of course). My understanding is that the IDs are what is used in the syncing configuration setup and as such the name that you see is not significant to it and can therefore be changed.
Thanks! I renamed the file and everything is working as expected now. I wondered whether renaming was the fix but was afraid to do it without posting the question as I thought I might break the link to Dropbox. I have to say, being able to use dropbox to securely share a moneydance file across computers running different OS's is SUPER! When I read about the dropbox integration a year or two ago I was skeptical that it would work. Now that I've set it up and I've used it, I'm a believer! I thought MD was great before the dropbox integration.....now I think you guys are genius!!!!