You need a driver for a usb-serial device.
Since there are only 2 brands of usb-serial chips it should be quite easy to get the AIS nmea
strings on you Mac native.
First have a look on the manufacturers page if there is an OSX driver.
The first driver I would guess you should try is the
pl2303 which can be found here: Products
Second option would be the fdti chipset:
Once you installed the driver you should have a look in Systemprefs->Network and see if there is a new interface. This interface cab be used in OpenCPN or other chartplotter software