The ins files are not "installed". They are imported from a Sonar menu. Here's from the manual:
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.
After that, you would assign midi ports/channels to the new instruments.