I think and recommend you should search around and read the stickies before asking questions which have already been asked 100s of times.
Please no one answer him so he can learn how to do a simple search, hopefully it will help him from asking further questions which have been asked already.
What client? Powerbot, simba, etc.
Keep in mind that you can always run bots on a mac by connecting to a vps or dedicated server via ssh. This allows you to pick the OS you desire.

