I created a file insync.list in /etc/apt/sources.list.d that has a single line in it:
deb http://apt.insynchq.com/ubuntu bionic non-free contrib
I installed the key as suggested on insynchq. After that, i was able to install the nemo plugin and run update. I’ve used the plugin to add and remove a few folders from within nemo. Everything appears to be working.
FWIW, here’s my system:
CPU~Dual core Intel Core i7-4510U (-MT-MCP-) speed/max~2386/3100 MHz Kernel~4.15.0-24-generic x86_64 Up~55 min Mem~3102.9/7911.1MB HDD~2000.4GB(56.5% used) Procs~235 Client~Shell inxi~2.3.56
But I did the same thing on a different laptop running Mint 19, and it worked, too.