I created cordova app on my local machine and uploaded to github and then linked to my phonegap build account
but I only see android icon for building the app . I dont see ios icon to click where I can build for iOS ( I have ios developer accounts and keys ready)
please advise what I need to do so that I can see the icon for ios
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).