You don't load the instr def file. It has to be setup.
Please read the section on this in the Sonar manual starting on page 1375. Once setup you should never have to do it again.
The below is from page 1379
To import instrument definitions
1. Go to
Edit > Preferences > MIDI - Instruments.
2. Click
Define to display the
Define Instruments and Names dialog box.
3. Click
Import to display the
Import Instrument Definitions dialog box.
4. Choose the file that contains instrument definitions for your manufacturer, and click
Open. SONAR displays a
list of all the instrument definitions in the file.
5. Choose one or more instruments from the list, and click
OK.
6. Click
Close to close the
Define Instruments and Names dialog box.
The instrument definitions you imported should now appear in the
Uses Instrument list in the
AssignInstruments dialog box.