Like all third-party soft keyboards, MouseCode Keyboard needs to be explicitly enabled after installation & selected as the current keyboard.
Install MouseCode from Google Play
Before Android will allow you to use MouseCode Keyboard, you have to explicitly enable it. The precise details will be slightly different for every phone. If you search Google for details, make sure your search terms include the name of your phone... "samsung galaxy note 5 enable keyboard", "nexus 6p enable keyboard", "HTC one m9 enable keyboard", etc.
Until you've successfully enabled MouseCode Keyboard and
used it at least once, you'll be presented with a button near the top of
the screen that appears when you launch the keyboard's control panel (via
the launcher icon). Clicking this button will hopefully take you
to the screen where you can enable the keyboard... but whether or not it
actually does depends upon whether or not your device's manufacturer
(or whomever made its ROM image) did it correctly. If it works, great...
but don't panic if it doesn't.
On most "Nexus" devices, you'll launch Android Settings, then navigate to "Language & Input". Click "Current Keyboard", then "CHOOSE KEYBOARDS". You'll be presented with a list of keyboards installed on the phone, and MouseCode Keyboard should be on the list. Click the toggle switch to enable it, then click "OK" in the warning that gets shown.
Go to Settings -> Language & Input, and click "Current Keyboard" again. You'll be presented with a list of currently-enabled keyboards. Choose "MouseCode Keyboard".
If you have a "Nexus" device is running Android 5 (Lollipop) or newer, you can also launch the "Current Keyboard" setting by doing something that causes the currently-selected soft keyboard to appear, then clicking the "keyboard" icon that appears on the right side of the softkey area at the bottom (usually) of the screen.
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