Download a fresh .jad file from the MXit download site (not the jar file!). Once you have the file, open it in you favourite text editor. (Right click, Open With, notepad/wordpad/etc).
Somewhere near the bottom will be a line that starts with 'c: 24' and has a large random string afterwards. You want this random string (always starts with 24 at the moment) later, eg:

c: 24c9bd1846-1f10-4632-adf4-bcfe1615aced1Q72Z112

You'll now need to connect to your jabber account via a jabber client the supports registration such as Psi or Gajim. You won't be able to use Pidgin/Gaim to register as it doesn't support registering new transports. However once registered to the transport you can easily use any xmpp/jabber client.

Open the Service Discovery window in the client, and browse to the server that has the MXit transport you want to use. If you double click the MXit transport then the registration window should appear. Fill in your details, eg:

Username: 27831234567
Password: kensentme
Misc: 24c9bd1846-1f10-4632-adf4-bcfe1615aced1Q72Z112

Once the transport connects successfully to the MXit service, you should receive a prompt to add the transport to your userlist, and all your contacts should start importing.