One of the key features of the SHMUP Creator is the in-editor testing of your game.
To play the game:
You can press the Tab key to start the game, and press the Tab key again to go back to the editor.
To play the game in the editor:
You can press P, or the spacebar to start the game, then press P, or the spacebar, again to stop it.
You can also click on the 
Play button on the right of the toolbar to start the game, and press it again to stop it.
In the editor, all the gizmos, helpers etc. are not hidden to help you debug your level.
Note: you can play the game in any camera perspective, which can be useful to debug your game.