If you want to do it via a GUI although you will still need to add the gpg key via terminal
The Easy Way
I add sources by typing:
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sudo gedit /etc/apt/sources.list
after which you type:
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Sudo apt-get update
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sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com <INSERT 16 DIGIT CODE HERE>
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sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com B530D6DCB5140445