The Android versions/releases of KOReader are still experimental. While it has worked fine on some devices running under some Android implementations, it may not work at all or not properly on your device.
Android port of the Koreader project is available now.
For the current nightly build, with the latest features.
The WiKi.
It appears that the current nightly may not function well on all devices or Android versions, so you may want to try an earlier Nightly release or
The Stable release.
Tap the top of the screen to bring up the menus. The same when in a document/ebook.
[Skip to post #3, for more tips]
T62/62+/C57ML users see
post 35.
The nightly referenced.
Update:
There is a lot of Android trouble shooting and fixing underway at Github, at the moment, there should be new versions soon that work on more devices and more reliably on some that were only marginal. The 20151027 nightly, published today 20151029, includes some changes that may improve Android compatibility.
Luck;
Ken