EndNote is available as an app for the iPhone and iPad. You do need to create an EndNote Basic account in order to use the app. Download the app directly from the Apple App Store.
You can synch your EndNote Basic library with the iPhone or IPad app, add and edit references, and annotate PDFs. You can't use Cite While You Write with the EndNote app.
There isn't an EndNote app for Android devices. You can use EndNote Basic with the MS Word app on Android following the suggestions found in EndNote in Word on Android or iOS.