You don't have to disable complete Windows Firewall. When you add computers to Notezilla (using Send To->Computers option from a sticky note), Windows firewall will automatically ask you if you want to allow Notezilla to communicate. You would need to add computers on every computer running Notezilla . Note that when adding computers, Windows Firewall should be enabled. Only then it will detect and add Notezilla .
First, make sure at least one computer is added in the computers list in Notezilla . If after enabling Windows Firewall and adding computers, you are unable to send note then you would need to go to the Firewall and configure the PC firewall to allow Notezilla .
You can search for firewall in Windows search box and you will see Allow an app through Windows Firewall . Choose this option and enable Notezilla . If Notezilla is not listed, then click on 'Allow another app' and add Notezilla . Also make sure that the firewall options to allow the app is enabled. If Notezilla is not listed then you will need to add it. Try all options Public, Private etc.
If you are using any other Firewall tool (usually included with Anti-Virus software) then you will have to configure that firewall.
Make sure you do this on all the computers.