If you are getting an message saying "This app is taking a long time to install.", then this means that it will take a while to install.
The App does take 10 to 15 minutes normally to install.
You will get an email confirmation after the install process has finished.
Or, on the Screenshot shown below, you can press the "Done" button and this will take you to the installed Apps page of Salesforce. From here, periodically do a control+screen refresh until you see "Payments2Us" as one of the Apps.
