Hi David,
Cart 4.67 isn't compatible with DNN 6. The first Cart to be release for DNN 6 was 5.0. Also you should never install the trial version over the full version of the Cart. The best way to test the latest version of the Cart would be to create a fresh DNN sandbox and install the latest trial version.
If you would like us to perform the upgrade for you so that you retain your data, we offer a Module Upgrade Service available from the following link -
Module Upgrade Service
Hope this helps!