Here are all important links:
http://lmms.io/forum/viewtopic.php?f=1&t=4740
-A few rules and useful forum instructions
If you like to introduce yourself, to the community, go here:
http://lmms.io/forum/viewtopic.php?f=4&t=4480
That must be a setting in your setup of your MAC-OS.
I have no knowledge in MAC at all, but in windows, you will get a similar fail and warning. In that case you need to scroll way out to the right where there is a 'hidden' button that let you Download anyway:
You should also log in as user with full rights -eg log in as what is similar to Administrator in Apple OS, if that exists 🤫❓