@ Gerald - from your message I'm guessing you just downloaded, unzipped, and then opened up some model library. If that is correct, then some installation steps didn't happen... please note the instructions in this submission (this submission and instructions valid for 12a)
To install support for Arduino Uno, type "targetinstaller" in the MATLAB Command Window.
In the Target Installer that opens, do one of the following:
• If you have not downloaded the support package, choose "Internet". Target Installer automatically downloads a support package and installs it.
• If you have already downloaded the support package, choose "Folder". Then specify the location of this support package. Target Installer installs this support package.
If you have further troubles - please feel free to contact Technical Support and we can get you up and running.
@ Kyler - Mac support for Arduino was added in 13b - if you have 13b, open up the .mlpkginstall file from this submission in your Mac MATLAB session - http://www.mathworks.com/matlabcentral/fileexchange/40312