seems to be a similar issue as in the other recent post:
What version of the Plugin are you using?
You should have v1.3.1 beta from
(Scroll down to my first post there.)
There was a change introduced in J3.1.3 iirc
thet required achange for the JFile include,
which is fixed in V1.3.1.
EDIT: since some folks are having problems with older versions
of the plugin, i just made V1.3.1 an "official release",
so you can find it easier by just using the Download link
from the menu on the left.
Also check the "Use jQuery" option in the Advanced Settings,
you may have to set it to "Yes".