Because Matlab's DDE-interface is undocumented, Mathworks feels free (and has every right) to change the DDE-behaviour between versions. In Matlab2013a they changed the DDE "Application Name" so that it now includes the version of Matlab. In this case, the correct "Application Name" is ShellVerbs.Matlab.8.1.0
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For reference, here is a screenshot of the settings (sorry, my Windows XP is in German.)