Can you confirm which version of ArtRage you're using? The .png exporter went through a change between 2.05 and 2.11
Note that if you export the entire image (from the file menu), it'll be against paper (in the latest version of ArtRage).
If you want it to appear transparent in the .png file or the .psd file, make the paper transparent before you export. If it's just a single layer you want to export, you can also 'export layer' from the popup menu next to the layer in the layer stack.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"