Tell Atlassian you have made a mistake and ask for a refund, as you have bought the wrong type of licence. If it's within 30 days of purchase and not a "starter" licence, you should be ok. Start at http://www.atlassian.com/company/contact
Then, buy a Cloud system (sign up for a trial, and then pay for it) separately, a Server licence is no good for Cloud.
(I don't know if they transfer funds)
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