Choose the Scan directory Tab and set up a profile to monitor C:\Users and 'Recurse into subdirectories,' if desired, then set the pattern for the type of files, e.g., *.dat etc. and then under alarm messages tab, set the 'Space used by files' in C:\Users or the 'Size of file' for individual files. Configure the alarm threshold "Watch" and set the expected value and operator for the alarm threshold. Set the alarm threshold low enough to test generating an alarm.
example alarm:
Users: Users size of file 10240.00 bytes, expected <= 3 bytes ({9D7E7D94-AF63-11E5-93FD-005056867C4D}.dat)