We would be happy to help you but please recognize that we cannot help you with your controller, you will have to tell us about it. We don't know much of anything about the EWI 5000 (?) so you will need to tell us about it. We can certainly help you with the Motif-Rack XS which we assume came with its Owner's Manual. And that you have downloaded the Reference Manual (PDF) and the Data List booklet (PDF) found on the official Yamaha download site: http://download.yamaha.com

There are two main modes on the Motif-Rack XS: Voice mode which allows you to play one sound at a time, and Multi mode which sets up the unit to play as many as 16 sounds at once on as many as 16 Midi channels. Additionally, mainly for keyboard controllers, there is a mode where 4 Parts can be triggered/played from a keyboard, some under direct control others triggered by an arpeggiator.

We do know an EWI is an electronic wind instrument but have no info on what it sends or is capable of sending. The Motif-Rack XS defaults to sounds being under control of velocity via Note-On events, it also responds cc011 Expression, cc007 for Volume. We imagine you are sending in via 5-pin Midi - make sure the Rack XS is set to MIDI IN/OUT = MIDI. If you are sending Breath Control, sorry none of the Rack XS sounds come preset for Breath Control, although you can program them to respond to cc002 and store the Voice to a User location.

As to why your Rack XS only has a piano sound is going to remain a mystery without more information from you. We assume you have read the manual on how to select Voices (?)
You can send in Program Changes or simply use the front panel to select from a wide variety of sounds
Preset: 640 normal voices + 48 drum kits
GM: 128 normal voices + 1 drum kit
User: 256 normal voices + 32 drum kits
(includes 128 Preset normal voices and 22 initialized drum kits)