Make sure that you have the dot "." before the "sbin". In your case the path to where the binaries are located will be is:
However, from looking at the port itself I do not see that the deskutils/xfce4-xkb-plugin port has any actual binaries. Instead, it is designed to be added to an existing installation of XFCE4. As such, I would recommend installing this one as a port install rather than as a PBI, that way it integrates properly into your existing installation of XFCE. I have added this plugin into our XFCE-Plugins System package for PC-BSD 9.1, so it should be easier to get this functionality once the next testing release for 9.1 comes out.
As for textproc/aspell, it looks like that port does have proper binaries (bin/aspell, bin/aspell-import). So the binaries to run in the PBI should be located in "/usr/pbi/aspell-amd64/.sbin/". Do *not* confuse this with the "/usr/pbi/aspell-amd64/sbin/" directory, as that one will not have any binaries because the port does not install any there.
I hope this helps!