For some reason xfce4-xfapplet-plugin has been dropped from recent Centos/Fedora releases, as the spec file for xfce4-panel proclaims:

# xfce4-xfapplet-plugin isn't in F15
Provides:       xfce4-xfapplet-plugin%{?_isa} = 0.1.0-11
Obsoletes:      xfce4-xfapplet-plugin <= 0.1.0-10.fc15

This makes it basically impossible for anyone to use a Gnome2 applet on Xfce4. As a temporary work-around I have built an RPM of xfapplet-plugin on Centos 6.3 with the release number of 0.1.0-15, which gets around the Provides/Obsoletes lines:

xfce4-xfapplet-plugin-0.1.0-15.el6.x86_64.rpm

The git repo is here: https://github.com/carlohamalainen/xfce4-xfapplet-centos6.