It's a false positive. If your virus software is prohibiting you from downloading and/or installing tModLoader, temporary disable it.
As for my own issue, I'm trying to enable
building and debugging through Visual Studio. Having done everything on that page, I keep running into CS5001 (no Main method).
My assumption of how this is supposed to work (I've never done something like this before) is that VS would execute Terraria.exe's Main method (or tModLoaderServer.exe, according to that page), but I don't seem to be able to communicate that to VS. Am I missing something?