You need to make sure your function can be found by Matlab by placing it on the path.
The general advice is to have a folder where you put functions like this (e.g. 'C:\Matlab\FEX downloads\'), and then add that folder to your Matlab path.
You can also select a web site from the following list:
How to Get Best Site Performance
Select the China site (in Chinese or English) for best site performance. Other MathWorks country sites are not optimized for visits from your location.