What do you mean by activating SNCB?

You can only activate a pass or a travel day and it's best not to do that in advance, only shortly before boarding.

All trains you use need to be added to your pass by tapping on the slider behind the train in My Trip. Adding the first train of the day will activate a travel day. You can only add a train to your pass after you've activated your pass, only then the slider will become visible.

I guess it is to get our tickets. I did it for ÖBB. I have purchased passes from Paris to Germany and Brussels to London with SCNB. There are specific dates attached to them. Do I need to activate them in the app or once purchased they are ready to go?

Also, can I find a past conversation I’ve had on here?

Just tap on your own name to see all your conversations.

The rail pass from Eurail is your ticket. You need to add every journey in the app and activate them. But that can be done "last minute". It has to be shown when asked for.

All the other "things" from SNCB or ÖBB are reservations.

You need to show your reservation at the station or to the conductor just when it's a mandatory reservation. So just have them with you. They are not linked to the app. Just print them and show it when asked for. You'll need them to find your reserved seat anyway.

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