Hi,
If I understand things correctly, the minimum requirements for Cubase SX 3 are a Power Macintosh G4 and Mac OS X 10.3.3. If so, that means that the program cannot be used on a Power Macintosh 7600/132. That would also explain why the aforementioned file is a .dmg (a disk image type used with Mac OS X).
You may want to contact the manufacturer Steinberg for information about whether another program/version for Mac OS 9 or earlier is available.
If you just wish to transfer files on the 7600/132 to the new iMac, Ethernet can be used locally (TCP/IP, via a router with a switch). "Normal" file sharing does not work because of operating system incompatibilities. However, you could install a small FTP server (such as the now free NetPresenz) on the 7600/132, and then access the files using an FTP client on the modern machine. Alternatively, you could use a "cloud drive" on the Internet. The Danish System 7.6 should include basic TCP/IP software. It should be possible to use a CD-R for any needed transfers of files (like the FTP server software) to the 7600/132, unless that computer is capable of connecting directly to the Internet.
Jan