IT IS in the 'Install device' button list, at least on 6.5.4. There are even three Motif Rack related items : check again...
Otherwise, and in the case that 'Yamaha MOTIF-RACK (Yamaha)' / 'Yamaha MOTIF-RACK ES (Yamaha)' / 'Yamaha MOTIF-RACK XS (Yamaha)' are all truely missing, and that you can't retrieve your previous device definition file, you are either for :
- choosing one as close as it and edit it as needed,
- defining a new one : a tedious task but doable. Choose 'Define new...' in the list, set the correct parameters in the 'Create new MIDI device...' window and use the 'Device' and 'Patch banks' panes to set everything as you have it on your gear. In the 'Patch banks' one, don't forget to tick on the 'Enable edit' box.
DAW : i7-870 (HT enabled) - Asus P7-P55D-E - 16 Gb DDR3-1600 - 2 x WD Black Caviar 1 Tb - Radeon 6450 GPU (512 Mb) - Windows 7 Pro SP1 (64) / E-Mu 1616m PCIe + Behringer ADA8000
Soft & plug-ins : Cubase 5.1.1 (64) & 6.5.5 (64) + HSO / Emulator X3 / TruePianos / Ivory II / GSi Mr Ray73 MkII & VB3 / Sylenth1 / Alchemy / Loomer String / Arturia MMV / BFD2 / Ariesverb
Gear : Studiologic VMK-188+ / Akai MPD32 controller / Akai ME30P MIDI Patchbay / Korg 05RW / Roland D110 / Yamaha TX802 / Zoom G9.2tt / My really old ones (pre-MIDI era) : ARP Odyssey + Korg MS20