There are two components to running Dradis: the web app and the background worker (that processes the uploads). It seems that your background worker isn't running (and I've just checked the docs, and we make no reference to that there, so thanks for the pointer, and we'll make sure to update the info).
To get the background worker running:
bundle exec rake resque:work
That should do the trick.
Alternatively you can process the files directly through the console, using Thor as described in [i], for a Nessus or Nexpose file, the tasks you'll need are:
bundle exec thor dradis:plugins:nessus:upload /path/to/file
bundle exec thor dradis:plugins:nexpose:upload /path/to/file