In this page of dotCover options, you can adjust unit testing settings related to NUnit tests.
Enable NUnit support
By default, dotCover adds action indicators allowing you to execute NUnit tests right from the editor and automatically discovers NUnit tests in your solution. Clear check boxes corresponding to specific NUnit versions to disable this behavior.
Custom NUnit installation
Use specified custom NUnit
If you want to use another NUnit version, tick this checkbox and enter the path to the local NUnit installation into the corresponding field
Load NUnit Addins
If support of NUnit 2x is selected above and you need to use NUnit addins for this bundled version, put the addins in the NUnit2\addins subfolder of the dotCover installation directory (which is shown for your convenience) and choose how to load them.
When running tests, set Environment.CurrentDirectory to:
Starting from NUnit 3, current directory is not set by the NUnit framework (see this answer in the nunit tracker). dotCover sets the current directory to the temp folder by default, but you can change it.