How do you open aspx files on a Mac?
'open' in what respect?
In simplest terms they're just text files, so any text editor will do.
However I suspect you really mean how do you execute the code contained within the files, right?
In that case, you don't.
.aspx files are associated with Microsoft's .NET web environment - their proprietary alternative to Java, WebObjects, and other web technologies. It only runs on Windows servers (with some minor caveats via an open-source attempt to port the technology to run under Apache).
As a result, the only way to run the code is to install the .apsx files in a Windows server and access that server via your web browser.