How do I open an .aca file?

You refer to the content of aca files using specially crafted URIs in markup/script e.g.

file:///hddvddisc/ADV_OBJ/myfile.aca/foo.jpg

i.e. treat the aca file as if it were a directory.

Sorry, I misunderstood you. Windows has no concept of .aca files so cannot decode them.

I thought you were wanting to consume them from HD DVD code.

The CreateACA tool shipped with iHDSim does not do any obfuscation; an ACA is simply an archive file like a ZIP file (but without the compression). If you opened it up in a binary editor, you would see all the raw text after some header information. If you have the HD DVD spec, it would be fairly trivial to write a de-ACA program.

If you have an AACS-protected disc, then you have the option of encrypting the script code (both inside and outside of ACAs).

Yes, in that case they have used a different tool and have AACS-protected their script. The XML is only protected against tampering (modification), but the script is also protected against viewing (so you don't copy their code).