QTCheck standalone app on Mac
If you wish to download, install and run QTCheck on your Mac as a standalone application (i.e. if you do not have a copy of Max/MSP) you will probably need to change your security settings. This is because your Mac will notice that it has made by an 'unidentified developer', and by default will prevent you from opening it in case it does something harmful to your computer (it doesn't!). When you try to run the application, an alert will pop up telling you that the file is 'damaged'.
If you have OSX El Capitan (10.11) or earlier
Open System Preferences, and go to Security & Privacy > General. Click the padlock to allow you to make changes (you will be prompted for your administrator password), and then under 'Allow apps downloaded from' select 'Anywhere'. You should now be able to run QTCheck.
If you have OSX Sierra (10.12) or later
When QTCheck has finished downloading you will probably see a warning straight away advising you to discard it. Reject this.
Changing security settings on these more recent versions of MacOS is a little more complicated (the 'Anywhere' option on your security preferences is hidden by default) but has the same effect. Open Terminal, and type:
sudo spctl --master-disable
You will be prompted for your administrator password. Once this has been entered you should be able to run QTCheck. (If you go to System Preferences > Security & Privacy > General you should see that 'Allow apps downloaded from' is now set to 'Anywhere'.)
Once QTCheck has run once, you can change your security settings back without affecting it. To do this, return to Terminal and type:
sudo spctl --master-enable