After you downloaded and installed the DMM app, open it and press the top left button(red circle)
This window will pop up, press the yellow button(red circle) to register if you don't have an account or the white button(blue circle) if you have one.
If you are registering an account scroll to the bottom and choose one of the login options, google/twitter/facebook.
If you pressed the white button(blue circle) to log in with an existing account this window will pop up.
After you logged in click on the controller icon at the top right(red circle)
If you just registered then keep scrolling down until you see the game icon on the right. Click on it to go to the game page.
If you have played KanColle on PC you should find this game icon in this window. Click on it to go to the game page.
Click on the yellow button to download KanColle(you don't have to check the 2 options in the red circle)
After the download and installation is complete the game page will look like this. You can now play KanColle by pressing the yellow button or using the Kancolle app.
P.S. the download page isn't correct because I took the screenshot after I downloaded the game, But it looks exactly the same as the game page without the gray button under the yellow button