One of the issues you may have when installing an application, created, and signed, using Eclipse:
Things to check:
Are you using a compatible Android device?
You will have problems putting an app made with Android 4.0 on an Android 2.3.5 device.
Check the AndroidManifest.xml for the miniSdkVersion!
Lower it to “3” and try recreating the apk
<uses-sdk android:minSdkVersion=”3″ />
If these things are not the issue, try installing the apk on a different device – rule out the problem either being the device or the apk you are creating.