Import repository gpg key. For example for [http://download.sugarlabs.org/packages/SweetsDistribution:/0.94/Ubuntu-11.04/ Ubuntu-11.04] repository, type in a terminal:
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Register repository with importing its gpg key. For example for [http://download.sugarlabs.org/packages/SweetsDistribution:/0.94/Ubuntu-11.04/ Ubuntu-11.04] repository, type in a terminal: