After using WWMT, some people find that they cannot export an SMD manually through File > Export. This problem happens because an INI file that tells the Wunderboy SMD Exporter to run in batch mode doesn't get deleted after export in some occasions (either because of file-lock or an uncaught exception).
You can alleviate this problem completely by updating your SMD Exporter to Version 1.6 or later at www.wunderboy.org (http://www.wunderboy.org) .
You can also read the relevant entry on the WWMT FAQ page on how to manually fix the problem (http://wallworm.com/projects/utilities/docs/resources/faq.html) (just delete the INI file).
Of course, just updating the SMD Exporter to 1.6+ is the best solution.