I recently upgraded to 5.3.1 version of Unity and I noticed that I now cannot build scenes or even switch platforms to anything and the one I wanted to switch to was Android. I have an android device that I plug in to push the application to but it just says No Android module loaded.
same problem for me. I’ve found bad news from Unity and hope they solve it soon.
http://forum.unity3d.com/threads/unable-to-select-android-platform.372097/
Hope this help you.
STEPS TO SOLVE ANDROID BUILD PLATFORM PROBLEM.
Step 1 : Download the unity download assistant from the patches or unity installer.exe.
Step 2 : while running the exe select the android build support, also you can unselect or select the rest build support with the installer file.
Step 3: after selection just continue installing,
It was easy then you thought. Cheers"!!