Licence on multisite

I want to use the RSS on multisite. 1) First I installed the plugin on the mainsite which is www.chanson.de. 2) Then I activated the plugin on one subsite www.zachze.de and tried to register it there - but that does not work. I got this message "The Echo RSS plugin's license is not activated. Please activate the license to use the plugin." 3) So i went back to the mainsite "chanson.de" to register it there, but there i get the message that it is already registered to the subsite and I have to revoke it. Please help!