Turn on Samba filesharing for the directory containing your media files on your mac.
http://docs.info.apple.com/article.html?path=mac/10.5/en/8201.html
In XBMC go to videos (or music), / add source / Browse / Windows network (smb)
Your shared directory should show up there- usually as 'Workgroup/mycomputername/mysharedfoldername' - select it and press ok.
In the dialog give your source a name - 'my media' or whatever. Press ok.
Choose what kind of videos / media is in the folder in the next dialog - TV / Movies / Music / Music Videos. Stick to the standard 'scrapers' for your choice for now and press ok.
Your source should now be in the files list- select it and you can browse your media in 'files mode'.
For the real good stuff search the wiki for info on Library mode:
http://wiki.xbmc.org/index.php?title=Video_Library