Android Studio automatically hides certain directories to reduce the load of indexing all those files. If you look in your module's iml file (e.g. root/main_module/main_module.iml), you'll see this line:
<excludeFolder url="file://$MODULE_DIR$/build/apk" />
If you remove that line, it will start showing root/main_module/build/apk/ in the file tree.
Unfortunately, Android Studio is smart enough to notice that it's missing and will add it again when it builds, so it's not really worth doing.
You can see a little more of the conversation I've had with one of the Googler's on the Android Studio project here: https://plus.google.com/104239612059033457922/posts/RVq9e9Viux5