I see that you are using Mac Os 8.6 which is very different from Mac OSX, what I use here (hubby has the Ibook, actually).
You really should give more information regarding your exact download problem and any error messages. A link to the problem downlad would also help.
I'm not a Mac expert by any stretch but here are my suggestions.
In OSX, the entire Netscape application is compressed inside a single .app "file" so you don't see individual files such as compreg.dat that you can easily find. So, the best method of replacing the program files would be to delete the Netscape 7.0x app from your hard drive, and then reinstall it.
Other download problems could be a profile issue, such as corrupted downloads.rdf file or an errant helper application (mimetypes.rdf file). Both can be renamed or deleted with Netscape closed and new, empty versions will be regenerated
If removing the entire Netscape application and reinstalling doesn't help then I would either delete or rename the downloads.rdf and mimetypes.rdf files from the profile's xxxxxxxx.SLT folder , OR, create a new profile for testing purposes. In OSX, there is a Profile Manager icon in the Application folder for Netscape 7.0x, or you can press the Option key while you double-click the Netscape icon.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.5) Gecko/20041217