|do the following command to find the file...|
dir sysdm.cpl /s
Take note of where the file is then set the path to include that directory with the following command...
the ; will append the directory to the path. Now try to reinstall. If this does not work you will probably need to run it on a Windows 98 box because the driver is too old to support current OS. I would simply install a Windows 98 Virtual Machine.
Note: Not tested, I didn't even know they still made ISDN modems. That tech is way old.