Okay, so pI3 can read your footage, but pIRender can't it seems. Sounds like a bug in pIRender, but can't be sure.
The 4:3 issue is probably because the footage uses non-square pixels, and particleIllusion only uses square pixels at this time.
Your best bet will be to convert the footage (using another app) to another format that uses square pixels and a different codec that pI supports. Then use that footage in pI. If the quality after converting it is bad, render just the particles with alpha in pI, then composite onto your original footage in your video editing app.
Alan.
....................
Alan Lorence
Product Manager
GenArts, Inc.
http://www.genarts.com
http://www.wondertouch.com