gdhgdh, perhaps I was a bit short in my explanation ... did the adapters come with any sort of packaging specifying modem use? Unfortunately for cables, if it looks like a duck and walks like a duck, brother it isn't always a duck. Like cables the adapters can be specific for printing, or for modem, or even wired straight through for use with whatever type of cable you have.

What I and others are saying is that the problem most likely lies with the cable you are using with your modem. A standard PC/serial cable will not work work, even on a PC. You still need a special modem cable. A standard data/printer cable will not work. Assuming you are using a PC/Serial modem cable known to work on a PC, then you have the added problem that the adapter you bought is designed for data transfer, which uses different pins than for a modem. Mac to Serial adapters are usually customized application, so it is unlikely your adapter is simply wired straight through, but rather designed to adapt the Mac's non-standard serial port for a specific PC serial use.