The secret may lie in the seller's disclaimer:

'Please note that attaching a PC Serial 9D device to a Mac computer via this adapter provides no guarantee that the device will provide complete functionality - or actually even work at all. Using this adapter as a part of PC / Mac Data Transfer solution will generally prove more successful (additional software may be required).'

You need a DB-9M/MiniDIN-8M cable or adapter made for Macintosh. They are no longer common, but I bought a couple from the French eBay seller salut59 last year.

http://pinouts.ws/db9-rs232-pinout.html

http://www.interfacebus.com/Apple_RS232_DB9_Pinout.html

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