There are a few reasons for this, the most common ones are:

• WiFi is enabled on Mac. This can cause the Mac to send out the discovery packets to find the DMXking device on the wrong network adapter. Try to temporarily disable WiFi while you configure the IP addresses for your set up.
• A lighting software is running. The Config Utility is using ArtNet to communicate with the network-based DMXking devices. ArtNet in its design is limiting communication to one application per computer, so when a lighting software that uses ArtNet is running at the same time as the Config Utility then the last software to be launched won't be able to communicate with ArtNet. When running the Config Utility make sure all other lighting software are closed.
• Firewall is blocking the ArtNet communication. Make sure all firewalls are temporarily disabled.
• Incorrect IP configuration like wrong subnet mask, IP conflict (all devices need a unique IP address), wrong subnet (192.168.0.x vs 192.168.1.x as an example) or multiple network adapters using the same subnet (for example 192.168.0.100 on one adapter, 192.168.0.101 on another adapter).