How strange... I've installed it on a 64-bit and 32-bit machine with no problems...
I tried /usr/bin/arista-gtk from the terminal, does not give a lot of output, just:
** Message: pygobject_register_sinkfunc is deprecated (GstObject)
Also tried running it through pydb (python debugger) but that was no help either.
Using lsof, determined that no additional files seem to be opened when the dialogue box for Create Conversion is opened...
I'll assume you downloaded the arista-0.9.7.tar.gz archive rather than from trunk (which is what I did - use the archive that is).
Perhaps there's a dependency missing?
I've run yum list py* and attached the output as text...
It's a long shot but you could compare against what you have installed and see if anything jumps out at you?