Wanted to let you know I had the same exact problem. Luckily I Googled it, found this post and was able to resolve it. I mistakenly thought the Ejunkie ID requested in the General Settings of the WP plugin was the email address I use to log into my Ejunkie account (it doesn't say ID number, just ID). I had no clue that there was an actual ID number assigned by Ejunkie.
I see that it is mentioned here: http://www.e-junkie.com/ej/resources.htm, but by the time I had uploaded the mp3 file, installed the plugin etc. I forgot I ever saw that.
Since the time we need to know where to find our Ejunkie ID number is when we're setting up the plugin in Wordpress, perhaps the best solution would be to ask the plugin developer to add a note below the Ejunkie ID field stating it can be found in our Ejunkie account in Seller Admin > View Account Summary. Also asking him to name the field Ejunkie ID Number would help clarify that it's a number, not a user ID.
I'd hate for people to give up because of something that can be so easily clarified!
Hope that helps