Hello Steve,
We are basically left with two simple questions here, apart from what Ken already mentioned. First, what type of plugin are you trying to install and what version is it compatible with? If it belongs to older versions of moodle, I am afraid, you are bound to get errors. Secondly, it is possible that your upgrade from the previous version has not been successful, in which case you would not be able to install any plugin as well. Since you have already tried uploading the required plugin via FTP, chances are that you are probably in a fix with either of the two situations I mentioned. Try and address these, and you might have your site up and running again, in no time! You may contact me if you need further assistance at admin@dualcube.com.
Regards,
Diptaroop