Not sure where you are stuck at, but if you wish to evaluate GreenHopper OnDemand, please ensure you have configured your OnDemand subscription in https://my.atlassian.com - click "Configure" link to proceed.
If you want to know how to start using it, take a look at http://confluence.atlassian.com/display/JIRA/GreenHopper+for+JIRA+Guide
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