You can log into your account on your device via Settings > Store if it's on iOS 5 or below, Settings > iTunes & App Store if it's on iOS 6+. If you want to it to appear in the 'manage devices' section of your account then you need to use iTunes Match or automatic downloads on it, or redownload some of your past purchases on it
Associating a device or computer to an account : http://support.apple.com/kb/HT4627
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