The problem is that old Macintoshes use a DIN-8 connector for their serial line, as opposed to the DE-9 (or DE-25) used in PCs. Fortunately it is possible to connect the two by properly wiring a conversion cable, and that's what I've done today. My first attempt failed because I built a DTE-DCE cable (used to connect to modems and other communications equipment), but in the end I got it, which resulted in a "null-modem" cable to connect the two machines.
Here is the scheme I used:
|DIN-8 (DTE)||DC-9 (DCE)||DC-9 (DTE)|
N/C stands for "Not connected". Use at your own risk.