Just had a nasty 30 mins fixing this one. Seems the package include te x86 processor DLL's but we run with x64. No biggie. Just need to download the latest ELMAH DLL's for x64 and drop those in.

Additionally the version of MySql.Data.dll used was different to that used by Umbraco so needed to add this to the web.config:

<runtime>

  <!-- Other dependancy elements here -->
      <dependentAssembly>
        <assemblyIdentity name="MySQL.Data" publicKeyToken="c5687fc88969c44d" />
        <bindingRedirect oldVersion="5.1.2.2" newVersion="6.1.3.0" />
      </dependentAssembly>      
    </assemblyBinding>
  </runtime>

To get the right newVersion value right click on the new MySql.Data.dll and select properties -> Details. The publicKeyToken should be given in the horrid stacktrace you'll be getting too or in the Event Log if you get the same issue.

Hope that helps someone.