Why the Artlantis Media Converter don't let me to converter an object?
Into the right column of the Media Converter make a right click on an object underscored in red, then in the list choose "Show error", very often it has to do with one or more missing textures.
In this case please do so: Open the .aof file into Artlantis 5, when opening it, a dialog lists all the missing textures, now you can either way delete these missing texture or associate them. Once done save the oaf file. Then use the Artlantis Media Converter to convert it with no problem.
Mac OS 10.6.x
Mac OS 10.7.x
Mac OS 10.8.x
Mac OS 10.9.x