By default, Sibelius 5 Essentials uses Kontakt Player 2, which is a 32 bit application. The reason you're probably not seeing it in Sibelius 7 is because you're opening Sibelius 7 up in its 64 bit version. 64 bit Sibelius will only show you 64 bit playback devices; 32 bit Sibelius will only show you 32 bit playback devices.
I'm not 100% certain it would work, but the first thing I'd try is to see if Sib 5 Essentials will load in
Kontakt Player 5 (is a free download). This can be run in 64 bit mode (32 bit too, but the goal for Sib is 64 bit).
If that doesn't work, then you may have to open Sibelius 7 in 32 bit to get KP2 to show up.