On your mac be sure to click on "options" when you select File Sharing and make sure that Windows/SMB sharing is checked off. Right now XBMC doesn't have native Mac Sharing (called AFP), but it can do Windows (SMB) sharing just fine.
When you are at the "Add Video Source" window:
Select "Browse"
Scroll down and select "Windows network (SMB)"
Select "WORKGROUP"
Select your mac from this next list
If you want all of the folders/shares on your mac just select "OK" (press left arrow to reach it) while you are looking at the list of options after you selected your mac. If you want a specific folder/share only then select/open that folder and select OK. You won't see individual files at this point, only folders/subfolders.
After you have selected OK you can change the name for this source if you want. Select OK
If you want to use the library feature (pre-downloaded thumbnails, plot summaries, etc) you can set it up at this point or just select cancel and set it up later. Either way you should be able to access and play all of your shared media at this point. If you want more information about setting up the library mode then let me know or check out the XBMC wiki manual.
One warning though, this currently does not work if you have upgraded to Mac OS X 10.7 (Lion). In Mac OS X 10.7 SMB is basically broken as far as XBMC is concerned and another method has to be used. If you don't have 10.7 yet then you don't have to worry. If you do then check out this thread for an easy alternative:
http://forum.xbmc.org/showthread.php?tid=106038
thank you
thank u soooo much for ur help 
