Infinit uses the built-in notifications system on Windows. When you see a notification for an incoming transfer above your system tray, at the bottom right of the screen, you can click it to open the app.
From inside the app
If you missed the notification, don't worry. Click the Infinit icon, to open the main view in Infinit that displays your transfer history.
Any row representing a contact for which you have unaccepted transfers will have a white background as well as two buttons - a green icon with a check mark and a grey one with a cross. To accept the transfer click the green icon.
The transfer will connect and then download. A progress bar will appear around your contact's avatar in the main view representing the time remaining for that transfer.