This is also a no brainer, at least once you realize that your "radio" is the combination of both your PC and the hardware that you buy from Apache, and that all CAT connections come from the PC, not the Apache hardware. Indeed, most of the "radio" is in the software, not the hardware.
1. Provision a TTL serial port on your PC. These days that usually means buying a USB to TTL serial adapter, like this
example.
2. If necessary, add a proper 3.5mm TRS connector to the adapter as shown for the Kenwood serial port configuration on Pages 48-49 of your KPA1500 manual.
3. Set radio type to Serial as shown on Page 32. The default Serial Speed XCVR baud rate should work fine.
4. Go into Setup > CAT Control, choose any of the four CAT instances available to you under the main CAT tab or CAT+ tab (they are all equivalent), and assign it to the COM port and baud rate your provisioned for your KPA1500.
That should be sufficient to get simple band following working.
In addition to this CAT connection, you can achieve full remote control of the amp on your PC by putting the KPA1500 on the LAN and using the remote control software provided by Elecraft and described on Page 35 of the manual.
You do not need DDUtil unless you wish to use certain DDUtil specific features. If that's the case, then you still need to do steps 1-3 above. Step 4 is changed to have PowerSDR or Thetis communicate with DDUtil via a virtual serial cable, then DDUtil connects to the hardware serial port to your KPA1500.