This issue has been resolved, thanks to Mathworks and my local IT group. For the record and for others who may experience the same issue, here's the workaround, which basically copies the MATLAB preferences file from the user who installed the Support Package to the new user(s):
1) Admin user installs Arduino for Simulink Support Package. Note that the package will not work for other users and requires this workaround. At least for MATLAB R2013a.
2) Same admin user runs attached MATLAB script file: Fix_Arduino_for_All_Pt1_Admin.m (in MATLAB, locate the file by changing current folder path -- the line above the Command Window -- to match wherever the file's been copied, and then right-click, Run)
3) Different user logs onto same PC and runs other attached script file from within MATLAB: Fix_Arduino_for_All_Pt2_Users.m This user should now be able to access and use Arduino for Simulink Support Package.