I am trying to add a text field onto my Android app in Eclipse, but then I drag the Plain text option on to the graphical layout, a message at the bottom comes
Check the "Android version to use when rendering layouts" and make sure you're not using a version that ends in "W" for Android Wear (e.g. API 20: Android 4.4W). I don't believe Wear supports EditText.
In both Android Studio and Eclipse, it's the dropdown with the green android in the layout preview's toolbar. You may want to set it explicitly and not allow the IDE to choose the Android version automatically.