If you are using Node import 1.x-rc4 or below with Domain Access, you can get this error on each row to be imported:
An illegal choice has been detected. Please contact the site administrator.
This error in this case is presented when Domain Access try to import a node without assigning it to a domain. Node import 1.x-rc4 and below lacks Domain Access support on 1.x-rc4.
Domain Access support will be available on Node import by 1.0 RC5 version, you have to dowload the -dev version to have it now.
After that, the error should disappear. For more information, read the first lines of node_import/node_import.inc , where this error is explained.