As far as I know there isn't an "out-of-the-box" keyboard shortcut to insert a new citation in LibreOffice Writer. It was discussed here:
http://support.mendeley.com/customer/portal/questions/451878-open-office-libre-office-plugin-shortcut
I'm running Mac 10.9.2, LibreOffice 4.2.3.3, Mendeley 1.11 by the way.
In LibreOffice:
Tools menu--Customize...; select Keyboard tab; find a shortcut you like (for my system ⌘^i has worked) and click on it; in the Category box select LibreOfficeMacros--user--Mendeley--MendeleyMain, and in the Function box, select insertCitationButton; Click the Modify button above; click OK. I closed the program and reloaded it, but I'm not sure if that's necessary.
One quirk: ⌘^ would be control + command keys by normal notation, but in fact, ⌘^i means when I press 'option'+'command'+'i' keys together the insert citation dialog pops up.
Enjoy!
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