If you're just looking to install WordPress for Android, you can find it on Google Play. If you're a developer wanting to contribute, read on.
Developer Blog: http://dev.android.wordpress.org
Source Code: http://android.svn.wordpress.org
Issue Tracker and Code Browser: http://android.trac.wordpress.org/
WordPress Mobile Developer Handbook: http://make.wordpress.org/mobile/handbook/
The full build instructions (including instructions for using Eclipse) can be found on the Tutorials & Guides page of the mobile developer handbook. Instructions for building on the command line with ant can be found below.
Once you have the Android SDK installed along with the library dependencies, run the following command from the root directory of the WordPress for Android project:
android update project -p .
This will create a local.properties file that is specific for your setup.
You can then build the project by running:
ant debug
You can install the package onto a connected device or a virtual device by running:
ant installd
Also see the full Android documentation, Building and Running from the Command Line.