Well, I'm at a bit of a loss I'm afraid. As you can see from the screenshot, the above method works for me.
I am using AVISynth 2.5- can you confirm you have this version installed?
If so, maybe your AVISynth installation is at fault? Try encoding with just the following in the filter editor:
Showframenumber()
This should produce video with a line of frame numbers hard-encoded down the left hand side. If this also fails, then I suggest you try to reinstall AVISynth.
Another suggestion is to select the "Force reencoding (for filters)" option under "Mode" and see if that has any effect.
Beyond that, it might be possible that there are sufficient differences between the free and Pro versions of DVD RB to mean that my method above will not work with the former.
Let me know how you get on,
G