The Modtool v4.2 is fairly outdated. I'd recommend either downloading Modtool 6.0 or 7.x (I like 6.0). None of the Modtool versions are capable of exporting directly into a Battlefront-readable format (.msh), but community tools have been made that support converting certain Modtool-exportable formats (.xsi, .obj) into Battlefront-readable formats.
See this thread
for more information.