You just add the trip to your pass from the Railplanner app :)

I see you're from the Netherlands, so "checking in” may mean “get through the gates".

If you have a paper Interrail, then the pass cover has a QR code for the gates. The mobile pass has a QR code after you've added a journey. If there are no gates, you just fill in the trip report (paper pass) or add a journey to your pass (mobile version) and you're OK. You cannot "check in” with an Interrail, just open the gates.

Hi, every travel day in your Pass has a different ticket, and each ticket shows all the journeys you add to your Pass for that travel day.

You can show your ticket in just 2 taps:

• Go to My Pass
• Tap 'Show ticket'

The ticket inspector will scan the barcode and check that the train you are on is listed on your ticket. They will also check your personal information and may ask to see your passport or ID.

I do not see a show ticket option! Though my passes say Ready to Travel 

Have you activated your pass?

https://www.eurail.com/en/eurail-passes/eurail-mobile-pass/getting-started