If you moved the MachFive, MOTU Symphonic Instrument, or MOTU Ethno Instrument DXi plug-in from the location it was installed, it's possible that Sonar lost the registered path. To reset the path:

Go to the Start Menu and choose the "Run" Option.
Type:
regsvr32 "PATH"

Place the path of the DXi Plugin where "PATH" is indicated above, including the quotes. For example, the default path to the MSI DXi is C:\Program Files\MOTU\MSI\DXi\MOTU Symphonic Instrument DXi.dll, so you would enter:

regsvr32 "C:\Program Files\MOTU\MSI\DXi\MOTU Symphonic Instrument DXi.dll"

You should get a message saying the plug-in was registered, and it should now show up in Sonar.