Maybe there is some other add-in in Excel 2013 conflicting with MySQL for Excel.
In Excel 2013, if you go to the COM Add-Ins page (File > Options > Add-Ins > Manage COM Add-Ins), do you see MySQL for Excel in the list?
If it is in the list, is its checkbox at the left checked?
If not check it and click OK. Open the COM Add-Ins page and see if it remained checked or if it was unchecked automatically.
Please read the blog post in the link below, and do step 3 to see if you can get the error message displayed to you and if so share it in another reply.
http://blogs.msdn.com/b/haahmadi/archive/2010/05/26/my-vsto-add-in-does-not-load.aspx
Best regards.
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Javier Treviño
MySQL on Windows Team