Thank you Very much Dan for looking into this .  I tried your command as well. But its failing with same error. 

My Command : 
zowe zos-files download all-members "DatasetName" -b -d "DirectoryLocation"

Error Message :

z/OSMF REST API Error:

Rest API failure with HTTP(S) status 404

category: 1

rc:       4

reason:   3

message:  DatasetName(

 

 

Error Details:

HTTP(S) error status "404" received.

Review request details (resource, base path, credentials, payload) and ensure correctness.

 

Protocol:  https

Host:      host

Port:      port

Base Path: basepath

Resource:  /zosmf/restfiles/ds/DatasetName(member1)

Request:   GET

Headers:   [{"X-IBM-Data-Type":"binary"},{"Accept-Encoding":"gzip"},{"X-IBM-Response-Timeout":"500"},{"X-CSRF-ZOSMF-HEADER":true}]

Payload:   undefined

Thanks,
Ajit