I wanted to use the Amazon app-store on my Google Nexus 7 tablet (mainly because I already have quite a few apps that I have bought for a previous Kindle Fire). So here is how to get the amazon app store on your Nexus device.
First, you need to enable installing of apps from stores other than the Google Play Store. To do this, go to Settings and select Security. Under Device Administration, make sure “Unknown Sources” is selected.
Second, install the Amazon App Store. To do this, down the “apk” file from Amazon from: http://www.amazon.com/gp/mas/get-appstore/android/ (There is additional information on Amazon’s page).