The Hardware Connection
Before you can even consider connecting your GPS to your Mac, you must have the hardware capable of making the physical connection. I'm talking about cables and, in some cases, cable adapters.
GPSes like these support one (or both) of two types of cables: serial and USB. If yours has a serial cable, I'm willing to bet that the end that doesn't plug into the GPS won't fit into any port on any Mac. It's a PC-style, DB-9 serial plug—the kind that fits into a PC's COMM port.
So that's the first challenge: getting the wire that goes from one device to the other.
For my older Garmin, which just has the serial port, I purchased a serial-to-USB adapter. I bought a Keyspan High Speed USB Serial Adapter (see Figure 1). But I'm sure that's not the only one that'll do the job. You can find comparable products by IOGEAR, CoolGear, CableMAX, and USBGear.
Figure 1 Keyspan is just one of several makers of USB serial adapters.
If you have one of these adapters, here's how to use it. Take the cable that came with the GPS and plug one end into your GPS and the other end into the adapter. (It'll be pretty obvious which end goes where.) Then plug the USB end of the adapter into a USB port on your Mac. For best results, use one on the Mac itself, not on the keyboard, or on a powered USB hub.
Keep in mind that you may have to install driver software to use the adapter you purchased. Read the instructions that came with the device.
If your GPS supports USB and came with a USB cable, you probably won't need an adapter. I say probably because not all USB connections work with all software, as I discovered. In fact, even though my Garmin GPSMAP 60C supports both USB and serial connections, I couldn't use the USB cable that came with it to connect to my Mac. I still had to go the adapter route. I hope that this situation will change as more Mac GPS software is developed, and hardworking shareware authors are properly compensated for their work.