All questions about using Cadabra can be posted on the Cadabra Q&A forum. The forum accepts LaTeX formulas and is fully searchable.
The old mailing list has been closed as it does not offer as much functionality as the new forum.
An extensive reference guide of version 1.x of the program, with a description of the input language, the algorithms and technical details on how to extend it by adding new C++ modules, is available as
preprint AEI-2006-038last version: November 2nd, 2012
An update is planned as soon as version 2.x reaches a stable state.