Well that's why I said to play around with it, check the manual, etc., it's not a simple as just adding a plugin and you're done.
For example if you use the one I did, in intelxdk at the plugins dropdown menu you have to click on "Get plugin from the web" and then fill with the following:
Name: AdMob Plugin
Plugin ID: com.cranberrygame.phonegap.plugin.ad.admob
Check the 'Plugin is located in the Apache Cordova Plugins Registry' box
P.S. And I think there was something else that even I'm forgetting.... rats.