As susan_cassidy said: "If it is truly constructed as a Kindle dictionary, it will show up as a choice when setting a default dictionary."
Your dictionary is clearly NOT formatted as a dictionary.
The "dictionary" folder, that is, the organization on the filesystem level that you see when plugging your Kindle in like a flashdrive, is insignificant. It has no effect whatsoever.
(This applies to all books, no matter their format or intent.)
(If your dictionary is in
Code:
documents/purple/please_dont_see_this/Harry_Potter_And_The_Sorcerers_Stone.mobi
it will still show up as a dictionary named something entirely different.)
If the book is formatted as a mobipocket dictionary, it will show up in the list of dictionaries available to your device. This will happen regardless of the name or filepath
all that matters is it
must have the right format.
There are freely available dictionaries that you can find in various locations online, or you can make your own. For help finding free dictionaries, or building your own, see the Sticky thread in this very forum, helpfully entitled "Dictionary FAQ".