In your case,
D:/Workspace/Project/build/classes
Is the directory where your IDE compiles the classes to. rebel.xml just specifies for JRebel where to load those classes from.
To use the relative path you can define a placeholder:
${workspace.path}/Project/build/classes
And then you can expand the placeholder by defining the placeholder value via VM arguments:
-Dworkspace.path=D:/Workspace
Unfortunately there's no way to specify the real relative path, a-la ../build/classes
My question would rather be why do you want to specify the path to JBoss's tmp folder?