It is very easy to sideload .apk files on an OUYA - you do not need adb or a file manager!
On your OUYA, go to Make > Upload and enable the browser-based uploader. Now enter the address in a browser window on your computer, e.g. http://192.168.3.34:10000.
You can drag and drop files from your file explorer onto that window, and the OUYA will simply install the games.
Note that the OUYA is running Android 4.1, so the app must be compatible with it.
