Each type of action is a sub class of the Action class. The subclasses have a property called Data which contains specific information for that type of action. As an example, if you want print all create card actions for a board you could do it like this: var board...
You can get all actions for a card this way: var card = trello.Cards.WithId(cardId); var actions = trello.Actions.ForCard(card); ...
This might help. I had the same problem of everything coming back null. I found that getting latest versions of certain components solved the problem. See this article: Trello.Net authorisation works but cannot access data once authorised...