Why rosmake does not work?

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Hi, I'm new and I tried to use rosmake like this

rosmake demo

But I get a problem

[ rosmake ] rosmake starting...

[ rosmake ] Packages requested are: ['demo']

[ rosmake ] Logging to directory /home/senna/.ros/rosmake/rosmake_output-20130203-110054

[ rosmake ] Expanded args ['demo'] to:

[]

[ rosmake ] WARNING: The following args could not be parsed as stacks or packages: ['demo']

[ rosmake ] ERROR: No arguments could be parsed into valid package or stack names.

No idea why. Waiting for help. Thanks.