DirectConnect with Pro Tools|24 works with Pro Tools v5.1 but only on disk tracks, which requires that the transport has been in play at least once before you'll hear audio (a limitation of the RTAS on disk tracks engine). If you set up everything but there's no audio hit play and then stop. Audio should be fine after that. Yes, beginning with version 5.1 DirectConnect will work only with MIX hardware or a Pro Tools LE system for the TDM and RTAS versions respectively. In Pro Tools v5.1 beta we determined that DirectConnect RTAS worked on disk tracks of a Pro Tools|24 system and that's why the 5.1 installer puts DirectConnect into the core plug-ins folder for a Pro Tools|24 software installation.