I assume you just install the drivers ( I don't know much about macs but there are supposedly drivers out there or even on the CD) and then just use it as a normal dial up connection.
The phone number to use when you are using it as a dial up is #99# (or *99#) I think. I'll check my PC downstairs which acts as my dial - up pc if thats wrong.
You need to use PPPOE (PPP over ethernet)
To do so you need to get the MyWireless ethernet cable which allows you to connect the MyWireless modem to an ethernet port. It should still cost about R250 and nowire.co.za possibly still sell it
The very first time you use the modem you need to set it up on a Windows machine using the USB cable. Just follow the usual MyWireless instructions and installer
Then set up the connection on your Mac. If you are plugging the modem into your router (e.g. a wireless one) then set up the router with the correct username and password for MyWireless, plug the modem into the WAN port and set your Mac to use Ethernet and DHCP in System Preferences: Network
If you are connecting directly from your Mac, Go to System Preferences: Network, click Ethernet and then click PPPOE. Tick Use PPPOE, enter your usename and password and you will be on your way
Vortex is correct in his instructions. However, a few things:
1) NoWire's outlets (Menlyn in PTA, Village Walk in JHB) DO sell the Ethernet cable which you WILL need.
2) Setting MyWireless up on a MAC with an operating system older than 10 will be a nightmare if it is even possible. In this instance I suggest you use a router and allow the router to establish the connection and then connect the mac to the router.
3) Also remember to change the MTU to 1458 (I think that is the correct setting - can't remember 100% right now).