To my knowledge the ink support is just for the ink controls and is only supported on Tablet XP or Vista.
This means you can create a picture control which can be inked upon or a blank control for a signature or drawing.
Other controls could be used with the hand writing recognition to add text but would not support inking outside of the one ink control.
I am not sure if you are trying to do something different with code or host the form inside of another app or something. Ink may not be supported under these circumstances.
You should be able to create a new blank template to test.
Add a field and ink control and then preview and ink upon it with the stylus. By default it will be in ink mode when completing an InfoPath form but it can be switched to regular mode in the toolbar.
Hope that helps.