You need to sign up for Twitter or App.net on their respective websites and use that login to add the account to Watercooler.

First, make sure you are entering your username and password for your App.net account, not some other account such as your App Store account, your email account, etc. If you need to, sign up for a new account using the instructions above.

If you are still not successful, you can go to the App.net website and use the Forgot Password link on the Log In screen.

You can use Watercooler on an iPad, but you will get the iPhone-sized screen put into a box on the iPad (with the option to zoom it large with a 2x button). A future version of Watercooler may add an iPad-specific layout.