I downloaded Absolution, got an executable and .wad file, put them in a folder, tried selecting the folder as an iwad folder as well as an add-on folder.
Restarting Doomsday won't produce the same crash upon startup, but it doesn't find Absolution either.
Doom64 Absolution isn't fully working in recent Doomsday builds. If you find an old executable, that will likely work best as-is.