How to press the middle mouse button in Xubuntu in VirtualBox on MacBook Pro?

I am running Xubuntu 14.04.4 LTS as a guest under VirtualBox Version 5.0.22 r108108 on a OS X El Capitan 10.11.15 host, on a MacBook Pro (Retina, 13-inch, Late 2013).

In Xubuntu, I want to press the middle mouse button (to paste the current selection etc). I use the trackpad, and I also use a Kensington slimblade trackball. How can I press the middle mouse button in this setup?

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