Steps

1. Log on as system admin (aka "host")
2. Go to Settings > Extensions > Existing > Libraries and install the CodeDom .net Compiler
3. Go through the installation wizard which appears, and you're ready to go

Important

If you have a Dnn that had been installed and upgraded many times (eg. starting from V7) you may have multiple similar looking entries in the list, like this:

In such a scenario, please pick the newest, and delete the others in the /install/library folder.