Here is some useful information from the experienced travellers in the Community regarding both planning, reservations and activation of pass and travel days. 

 Planning

The rail planner is normally not up to date, as it only is updated once a month, so to be sure of the time table you better check the timetable and availability on the websites of the national railways. The bigger national railways, like DB (Germany) SBB (Switzerland) and ÖBB (Austria) cover several countries. 

 Reservations 

I had now issue with the Railplanner app and adding trains to the pass Just keep in mind for activating traveldays and atleast every 72hrs you need a Internet Connection to keep your mobilepass valid :) 

As for tips/hints to make it easy: also seeing the endless daily listings here from people and these pesky RES-avoid those countries where they make it so difficult: ESpana#1, FRancia#2.

In IT, SE, NO, PL, RO you also need to RES any seat in longer distance, but thats fairly easy and can also be done locally @ counter. For IT also advance via OeBB, SE via the SJ site.

I myself have not yet used a mobile pass-that is due (from that sale last yr) in a few weeks now.

Most people that have some difficulties with the rail planner app are older, first time users. You seem quite young, so you won’t have any trouble with it. I find it quite handy, and it’s as quick an easy to use as filling out a paper pass. And you get a nice bunch of statistics too :)

The QR-code sometimes doesn’t open the ticket barriers (UK, Big stations in France (mainly Paris), Spain,...) But there’s always staff to help you in.

The only downside is the planner, in the app. That one is just not reliable. Times differ, information lacks sometimes. So always check with the railway company for the latest real time information. Or use the websites @AnnaB suggested.

Just use it to register trains, not to plan your journeys.

Whilst the app tries to emulate a paper pass it does need an understanding of the terms used and how the 4 elements of the app interlink.

First to sort out is the reservations section - This is simply a link to external reservation tools and plays no part in the pass or creation of travel tickets.
 

Next are the 3 sections - Planner, Trip and Pass.

The planner is both an overview of journeys and a method of adding planned trains to your Trip. It is a static/resident timetable and can be out of date (mainly by not showing trains) and should only be used to transfer planned trains when found. It only updates when a new version of the app is released. Planning is better done by using the operators own website.

The next section is called your Trip. This is simply a diary of your intended trains, but is the equivalent of writing a train into a paper pass by transferring the details of your intended train into your pass and creating a valid train ticket. Until you board the train you can remove these and add alternatives at will. If you cannot find a specific train operated by a participating operator in the planner you can add it manually to your trip before transferring to your pass.
 

Note - there is only one trip per pass for the full duration, even if you plan more than one excursion (What we call a “Trip”).

Finally there is your Pass which, not surprisingly, is your actual ticket for a rail journey. When you transfer a train from your trip (equivalent to writing it in a paper pass) the app creates a travel day and displays a QR code for that day. It also adds the train as a text list below the QR code. This combination is your ticket for that train. Subsequent trains on that travel day are listed below the QR code and again this is now a valid ticket. 

Note - If reservations are mandatory you will need proof of reservation as well as a validated pass, and equally if you have a reservation you will need a validated pass.