Once constructed you should use the get_collpane_win function to access the pane and add your controls inside it (i.e. use the GetPane's returned pointer as parent for the controls which must go in the pane, NOT the wxCollapsiblePane itself!).
Note that because of its nature of control which can dynamically (and drastically) change its size at run-time under user-input, when putting wxCollapsiblePane inside a wxSizer you should be careful to add it with a proportion value of zero; this is because otherwise all other windows with non-null proportion values would automatically get resized each time the user expands or collapse the pane window resulting usually in a weird, flickering effect.
Use the wxEVT_COMMAND_COLLPANE_CHANGED (/wxCollapsiblePaneEvent) event to handle the event when the user collapses or expands the pane. Note that you may need to handle this, and call layout_sizer in the event handler in order to get the sizers to properly resize.