See http://docs.phonegap.com/phonegap-build/configuring/#props ; look specifically at the "platform" tag section.
Now, check your config.xml file. Do you have any "platform" tags? If so, do you also have one for "ios"? If not, you'll need to add one, per the documentation above. Note: If you have zero platform tags, you shouldn't need to add any if you want to build for all supported platforms.
If you want to share your config.xml, you can (just strip any identifying information you don't want public).