According to the beginner guide, to setup the ADT Plugin, one of the procedures is
http://developer.android.com/sdk/eclipse-adt.html#installing
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The path to the SDK is:
C:\Users\USERNAME\AppData\Local\Android\sdk
This can be used in Eclipse after you replace USERNAME with your Windows user name.
a simple windows search for android-sdk
should help you find it, assuming you named it that. You also might just wanna try sdk
ADT Plugin (UNSUPPORTED)
The Eclipse ADT plugin is no longer supported, as per this announcement in June 2015.
The Eclipse ADT plugin has many known bugs and potential security bugs that will not be fixed.
You should immediately switch to use Android Studio, the official IDE for Android.
For help transitioning your projects, read Migrate to Android Studio.