Thanks 
CDogs1964 for pointing that out.
When I in 'intelxdk.config.additions.xml' add the following (in the end of the file):
<intelxdk:plugin intelxdk:name="https://github.com/cranberrygame/cordova-plugin-ad-admob"
intelxdk:value="https://github.com/cranberrygame/cordova-plugin-ad-admob" />
.. and I removed the 'cordova-plugin-ad-admob' plugin (via XDK Plugin Management), XDK was able to build the IPA (iOS package).
It worked for me, but I can't close the ads (no "x"). :s