Unfortunately xfce4-panel doesn't look for available panel plugins anywhere inside user home directory. And it doesn't have any settings/environment variables to control that either.
So the only natural way left for you is to recompile xfce4-panel yourself and run your modified version.
To make it easy for you I'll give you detailed instructions.
- Download from http://archive.xfce.org/xfce/ xfce4-panel sources (e.g. xfce4-panel-4.10.0.tar.bz2)
Unpack archive and run (and hope that you have all required dependencies):
./configure --prefix="$HOME"/.local \
Whisker menu doesn't require
--enable-gtk3, but some other plugins might want it.
Make sure that whisker files are in place (if not, you can just copy them) :
If xfce4-panel can't find your other plugins, copy them to
P.S. I also suggest you to ask your system administrators who have root rights to just install whisker menu on the system instead.