[plt-scheme] scribble sigplan

[is there a scribble-specific mailing list to avoid making so much noise on the plt-scheme list?]

From the scribble docs:

"1.5 More Functions

The scribble/sigplan and scribble/manual languages are supersets of thescribble/base language, which provides a collection of basic operations."

However, when I go to #lang scribble/sigplan, then @schemeblock triggers an unbound identifier error. 
Also, with scribble/sigplan, the nice PDF/HTML buttons disappear. 

Is there something special to do for sigplan to work? the Getting Started guide does not mention anything special.

Thanks,

-- Éric