There is a bug in Android 5.0 & 5.1 (completely beyond our control) where sometimes the Homescreen Widget is either missing from the list of Widgets or cannot be dragged from the list to the homescreen.
This normally happens shortly after installing the app.
The workaround is to restart/reboot your device. After this, you should be able to go to the list of Homescreen Widgets and add the Widget as normal.