Hello,
the addon collection was empty on my device too, I had to restart the Firefox Nightly app.

One step I forgot to say is to open the Addons menu in Firefox (three dots - Addons - Addons Manager)
There select the addon you'd like to install. There are currently two, this one and Get RSS Feed URL.

Also please make sure you entered the addon collection details letter for letter in Firefox.
Hope you can use it now:)

EDIT: If it still doesn't install (it should), the best solution would be to contact Mozilla about it. Depending on how technical you are, you can either file a bug report yourself, or talk to Mozilla support at support.mozilla.org