Hello,
I am not sure which component you use. I suppose that you want to use CAN_LDD (according to the name of the can_readFrame() method).
The CAN_LDD component help (open the context menu of the CAN_LDD component in the Components window and select Help on Component) contains Typical Usage page that contain examples - how to use methods of the CAN_LDD component, see below:

Best Regards,
Marek Neuzil