Installing the Developer Extensions can be a little finiky sometimes.
Here's are steps that will usually work to get them installed.
1) Close NT.
2) Try to install using the latest installer with Dev Extensions enabled (Run as Admin) - Will probably fail | If this step fails, BloodHound will be uninstalled.
3) Start NT, click on Tools>Edit NinjaScript>Indicator...
4) Select an indicator at random and compile it. No errors reported, move to next step. If you get errors, please resolve them before continuing.
5) Close NT without saving.
6) Install using the latest installer with Dev Extensions enabled (Run as Admin) - Success!
7) Back to step #4, recompiled random indicator with success.
If it finally installs and you get a compile error on step 8, there may be additional steps which is basically deleting a line from the config.xml.