I want to work with Rewire Cubase 5 (64 bit) and Sibelius 7 (64 bit), but don’t see “sibelius rewire” in the Cubase Devices menu. 
Giedrius
Covered a zillion time before!!!
Neither support rewire under 64bit! (Yet)
Although Sibelius can run in 64bit and communicate with a 32bit host, thus you would need to use C5 in 32bit.
Once C5 32bit, I can not use the library Eastwest (my 16 GB of RAM). Maybe this problem Steinberg will be solved…
Promised next update for C6, never going to happen with C5