Enable "Developer Mode" on Windows 10 build 10074

Here's the trick as currently documented here, as the method to do so through the settings window do not work (for me, Settings, For Developers crashes the settings application):

For Windows 10 desktop
Use gpedit.msc to set the group policies to enable your device, unless you have Windows 10 Insider Preview Home Edition. If you do have Home Edition, you need to use regedit or PowerShell commands to set the registry keys directly to enable your device.

Use gpedit to enable your device

1. Open a cmd prompt with administrator privileges.
2. Run Gpedit.msc.
3. Go to Local Computer Policy > Computer Configuration > Administrative Templates > Windows Components > App Package Deployment
4. Edit the policies to enable the following:
   • Allow all trusted apps to install (Enables your device for sideloading apps)
   • Allows development of Windows Store apps and installing them from an integrated development environment (IDE) (Enables your device for development from Visual Studio)
5. Reboot your machine.

Other methods to do so using regedit and PowerShell are found at the same link.