Do you know the old password?
if so, try entering it in BOTH the account Name and account Password areas at once to log in.
I believe the older v4 or v5 equivalent of the layer DDR would actually expose the old-style passwords (but you may have had to run it with full access). Been a decade since I last used it for that.
Plus 1 on that. And try different capitalization patterns. The original password was not case sensitive, but it is now.
The problem is probably that it is wanting the user name from the copy of FM it was made with.
so if it was made on FM with a user name of "John D Doe" and you current user name is "John Doe" it won't work.
You can't install FM6 on today's OSX.
If You have a VM (parallels, VMWare) with a Windows System, You can install FM6 there and check the files
under FM6 and older, there was only a password, no account name. During the conversion from .fp5 to .fp7, the account name was created - exactly the same as the password
You can leave the account name empty, just typing in the passwd. In some dialogs, both are required - enter in both the same...
the DDR will show accounts - therefore chances are good to find that
if You have set up an auto-login, just use the 'alt' or 'option' key while opening a db file - until the account/passw dialog appears
On a new system, the account name (FileMaker) is set to the OS user. Just replace that (empty...). You can define that name in the preferences of FileMaker
Once again I'm blown away by the resourcefulness and generosity of the FM community. Thanks to all for the useful suggestions. I DO have the original password, but I wasn't sure I know the account name, but @MarkusScheider's info on <v7 files lacking account info makes sense - no wonder I couldn't remember it. I have tried to access the DDR, but of course I can't get in unless I have admin access. I have tried various change-ups to the upper/lower case pattern, no luck with that.
BUT, I was able to borrow a USB DVD drive, and I do have Windows 8.1 installed via Parallels, so I installed v6 of FM there (frank evans for dual-platform install disks). I booted the DB up and tried the password - voila, she works. I saved a password-free copy. Problem solved. Thanks again for all the help.
I forgot that when I converted a file with a password that I couldn't work with it until removed it and converted non password file, even though I had all the right info on user name and password. so you did what was needed even though we whom have be through it forget sometimes.