Go to the Austiblox Directory > bin > data, and open up Roblox_Legacy_Place_Converter.exe.
Also make sure your .rbxl file is a .rbxlx file. You can do this by opening up the game in the latest ROBLOX Studio, then save it as a rbxlx file.
Once you're done saving the rbxlx file, browse for the file in Roblox_Legacy_Place_Converter.exe and then it will ask you to save the file as. Then click convert.