Yes, going to a showcase video should allow you to enter the code and have it work.
You should hear 3 dings if the code entry is successful (unless you have Tivo sounds turned off).
The TiVo Stream showcase video does not reside on the Tivo, it is downloaded. That's probably why it's not working.
You need to use a showcase video that has a film clicker thingy as an icon.
These are the current showcases I have:
Unlimited Digital Magazines*
Verizon NFL Mobile*
Ones marked with an asterisk all have videos that reside on the Tivo.