OK, I struggled with this for half an hour, so just to document the solution. On my new laptop with Ubuntu 12.04 and Libreoffice 3.5, I installed Mendeley with the deb package from the website. Installation went well, but when trying to install the openoffice plugin, it first complained it could not find unopkg. After some time I found that this was now located at /usr/lib/libreoffice/program/. So I pointed Mendeleydesktop towards this and now it ended with a warning that 'Couldn't Install OpenOffice Plugin The OpenOffice unopkg utility gave the following output:'. I read about manually installing the oxt which is at '/opt/mendeleydesktop/share/mendeleydesktop/openOfficePlugin/Mendeley-1.5.2.oxt' and opened the extension manager at libreoffice when I found that the Mendeley plugin is already installed. So, all I had to was restart Libreoffice and the Mendeley toolbar was now available.