On the Aura with firmware 2.5.1, the installation procedure given earlier in this thread for dictionary zip files only works partially, at least for installing the English dictionary that is not installed by default.
After copying the English dictionary to the .kobo/dict directory on the device, it becomes available for looking up words in a book, but if you tap the Dictionary lookup function on the bottom bar, it always says that there is no English dictionary available. If you switch the dictionary to another language using the dropdown list, enter a word, then switch back to English, it will successfully look up the word in the English dictionary. But it doesn't stick - the next time you call up the dictionary function, it will again tell you initially that there is no English dictionary.
Unfortunately in order to add dictionaries the "approved" way, you must also take the latest firmware update, whether you want it or not.