Radiusware is indeed the software required, but the problem is likely the monitor. It is usually necessary to experiment with the monitor settings (through the built-in settings of the monitor itself, not in MacOS) in order to get a relatively recent LCD to work with most any Nubus card or old Mac. A monitor that does auto-setup is ideal. Some LCDs may never work, and even in the cases of the couple that I have that are Mac-compatible, according to the user's manuals, successful use is usually not a question of plug and play.

The use of Nubus cards is also an issue to consider. I have had the built-in video of a Quadra work with an LCD when a high-end Nubus card in the same machine would not work with the LCD; in fact I had that problem with a Radius Thunder IV and an LCD, so the LeMans (which is closely related) may be doing the same thing in your case (though my screen was not a Dell).

Do you have an old CRT monitor to try with it, in order to test the LeMans card?