Hey Daniel,
In order to be able to run Typemock tests with JetBrains Rider you need to do the following steps:
Go to: C:\Program Files (x86)\Typemock\Isolator\8.6\BuildScripts\Scripts
Run mocking_on.bat script located in that folder.
This way you set the environmental variables to turn mocking on. After that, start Rider and try to run the tests.
You can experiment it with our Examples project located at the Typemock Installation folder. If SmartRunner is off for whatever reason, you need to set the environment variables manually: 
These values can be found in the script mentioned above.
