Hi David,
The error is related to loading the .dll library that handles the communication with the device. Can you please make sure that the library is properly installed by following the steps from the 'Downloads' section of this wiki page: http://wiki.analog.com/resources/tools-software/linux-software/libiio/clients/matlab_simulink?s[]=libiio
Also it is very important that you have latest Windows SDK installed on your computer. Matlab uses it to call dll related functions and this might be another cause for the error.
Thanks,
Andrei