You have an APK file, so I'm going to assume that you are trying to automate tests against a real Android application.
If so, Selenium is not what you want to use. This is an automated web application testing system. It will not work with native applications.
Robotium is probably the best you can use for this:
https://code.google.com/p/robotium/
Install it, go back and send a new question if you have any problems.
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