If you want to trigger the popups again, the simplest way is to delete the Tales of Maj'Eyal folder (in Program Files) and re-install. Saved games are held in another folder (under %USERPROFILE% in Windows) so should not be affected. Then rerun the GOG installer and start the game again.
The popups contained the following info:
* For Ashes of Urh'Rok, select the Age of Ascendancy campaign and choose the Doombringer (Defiler) class.
* For Embers of Rage, select the Ork race and choose the Embers of Rage campaign.
* For Forbidden Cults, select the Age of Ascendancy campaign and choose the Writhing One (Demented) class.
Ashes of Urh'Rok and Forbidden Cults add a lot of content to the main Age of Ascendancy campaign that can be experienced by any class (new items, monsters, areas, events and lore) so you don't need to choose those options straight away.
Two small FYIs I've run into:
* There may be cases where you run into problems and may want/need to replay sections of the game - consider taking regular backups of your savegames (e.g. every time you enter a new area).
* Tales has a level cap of 50 which you can easily reach. You can get add-ons to extend this, like
Adjustable Level-Up but these need to be present when you start a new character.