For version release 2017a and higher.
When using MATLAB in Windows 7 and trying to open FDTD solutions, you receive an error that it is:
Unable to launch the requested application:
To resolve this, add the application directory to your Windows 7 system path (requires admin access):
START> search for
"Edit system environment variables"
In the Advanced section, click the Environment Variables button.
Finally, in the Environment Variables window (as shown below), highlight the Path variable in the Systems Variable section and click the Edit button.
Add or modify the path lines with the paths you wish the computer to access. Each different directory is separated with a semicolon as shown below
..... C:\Program Files\Lumerical\2020a\bin; C:\Program Files\Matlab\R2017b\bin
More details on how to edit your System PATH in Windows (7, 8 and 10):