I've found out how to do it, following some guide.
This works now like it seems.
# Create the store object
self.store = self.builder.get_object("liststore1")
# Put stuff into the store
self.store.append(['1st column','2nd column'])
# ^ This is an example for a 2 column list store
But now I've got a new problem / question.
I've created a button called "saveButton". I want the button to run a function after clicked. As I'm doing this with Python and PyGTK I first tell the app who that save button is by doing this:
self.saveButton = self.builder.get_object("saveButton")
This should actually do something when being clicked but it doesn't.