If your antivirus is one of those listed in the preset options, try that one first.
You may need to adjust the path to the command if you installed the antivirus in a different directory.
If your antivirus is not one of those listed in the preset options, check your antivirus documentation for information on how to invoke command line scanning.
In both cases, open a command prompt and test the command that you would use in DeskNow , replacing %FILE% (used by DeskNow) with the full path and name of an existing test file.
If the command does not work from the command line, it will not work from DeskNow.
If the antivirus opens up a graphical window when invoked, then it will not work in DeskNow (the antivirus must support full command line mode, with no windows popping up).
Finally, check the log.Debug log file (it can be enabled in Administration/Logging to see exactly the command passed to the antivirus and its result).
Consider using one of the free antivirus alternatives that have been tested and proven with DeskNow.