This is likely due to the method Mike used to download the banner at the bottom of the launcher. Simply go into your settings and add an exception to Norton's checks for MM8BDM.exe.
If that doesn't work, try using DoomSeeker (http://doomseeker.drdteam.org/) under "create server" to launch sandronum.exe [included with MM8BDM's release zip] with the iwad set to megagame.wad and an additional pwad set to MM8BDM-v4b.pk3.
It should look similar to this:
(http://puu.sh/fECdx/ccddb54c56.png)
Then hit "play offline."
Or use ZDoom Launcher, but that program's UI is much more complex.