Dear Javier,
I am not aware of the internal structure of a truss, so I cannot help you with that without deeper exploration or further information.
However, if this is a specialisation of your more general query on how to access the internal components of a family instance, which I assume it is, then I can help you after all.
First a couple of question to you:
Are you pretty new to the Revit developer community?
Have you installed RevitLookup?
https://github.com/jeremytammik/RevitLookup
Have you worked through the getting started material?
http://thebuildingcoder.typepad.com/blog/about-the-author.html#2
Have you looked at some of the discussions on how to research a Revit API task or problem?
http://thebuildingcoder.typepad.com/blog/2017/01/virtues-of-reproduction-research-mep-settings-ontol...
Have you any experience with the Revit user interface?
That helps a lot to understand the programming side of things!
If you can answer yes to all of those question, let's continue.
If you answered no to any one of them, please complete that learning step before reading on. Thank you!
Now to address your question:
Have you explored how you can access the subelements of your truss through the user interface?
I would assume that you would do so by selecting a truss family instance and clicking the button saying 'Edit Family'.
The approach through the API is exactly equivalent:
http://www.revitapidocs.com/2017/56e636ee-5008-0ee5-9d6c-5f622dedfbcb.htm
I hope this helps.
Cheers,
Jeremy