Restore of a backup is currently not conveniently possible, but there are workarounds.
Install Firefox
Download version 2.19.5:
Restore your backup in this version of the extension
Update extension to current version from here: https://addons.mozilla.org/en-GB/firefox/addon/worldbrain/versions/
Login with the account you use and do the migration process you find on the dashboard
Wait until upload complete
If you have no other extension running, download the current Memex version in the browser you want to work in
If you have already an extension running, just wait until download in target extension complete
Done!