I have the same problem, and i found 3 different ways to solve this:
a - use another file manager, i personally use "Transmit" to do this sort of things;
b - use terminal, like: ditto <source> <destination>
. Or easier way just type ditto, and drag the source folder, then drag the destination folder, all inside source will merge inside destination;
c - unzip the plugin, inside the OC folder using the terminal, like: tar -zxvf plugin.zip
;