Have you determined where the problem is yet? I mean, is it the plugin you are using (which one ??), or Lightroom proper (or SmugMug or..).
For example, can you do every other thing in Lightroom which uses the internet? - like map module, or publishing to other services (if so, problem probably not Lightroom).
Does it work if you use SmugMug outside Lightroom? if so, then problem is probably not the SmugMug server.
Does it work if you use a different SmugMug plugin? if so, then problem may have been due to the plugin.
etc. etc. etc. (troubleshooting-101).
Has it ever worked? If not, maybe it's a setup (e.g. login credentials) problem.
Are you using the latest version of Lightroom, SmugMug plugin, and Operating System? (which OS?).