Fixed. . .The cause was having an SDK that didn't match the P3D version I was running. I fixed that by updating both my version of P3D and the SDK so they matched. That, however did not get the Traffic Toolbox working. Even with the updated info corrected in the dll.xml file, Traffic Toolbox would not load. I found a fix in the SDK Forum at P3D. Use the dll.xml file from the Traffic Toolbox folder in the SDK as a starting point, then copy and paste the remaining entries from the original into the new dll.xml. I don't understand why simply adding the correct version numbers into the current dll.xml file won't work, but it just doesn't.
Using this workaround fixed it though and that's all that counts.