I want to thank you as well.
After some tinkering I managed to make it work on my
Libra H2O.
Just adding the nickel menu was not enough unfortunately. The Dropbox button would just lead to an empty page...
It seems (on older devices) the link to the registration page is missing from the config file of the ereader. I had to add that URL manually, like
clisboa mentioned earlier.
If you have similar issues, check your readers config file under ".kobo/Kobo/Kobo eReader.conf", specifically the option "
dropbox_link_account_poll=". If this option is empty, you need to add this URL behind it:
https://authorize.kobo.com/{region}/{language}/LinkDropbox