I have the KVM version and am unable to Drag and Drop files between computers HOWEVER I am able to copy and paste between computers.
Noted in the link that basj posted:
First, Drag and Drop is not available in Multiplicity KM - only available in KVM and greater. Also, while Drag and Drop is available on on the KVM version, it does not work for KVM sessions, only Seamless. You would use Copy / Paste for KVM sessions.
Stardock Support Manager