It should have been registered by the installer, but apparently that didn't happen. You can do it manually as well:
C:\Microsoft.NET\Framework\v2.0.50727\InstallUtil.exe "C:\Program Files (x86)\SolarWinds\Orion SDK\SWQL Studio\SwisPowerShell.dll"
C:\Microsoft.NET\Framework64\v2.0.50727\InstallUtil.exe "C:\Program Files (x86)\SolarWinds\Orion SDK\SWQL Studio\SwisPowerShell.dll"
This registers it twice - once for 32-bit powershell and once for 64-bit powershell.