This is a very common question. I recommend a google search, site:stackoverflow.com your search term here to find if your question has been asked before. Also, Stack Overflow has a pretty good search, and you can always use that. Also, the "Ask a question" title box pops up a list of questions that may already have your answer. I know both of those questions are in that list.
OP, just a note: you probably don't need the client machine's MAC address. Call it me reading your mind. :). You might ask about the higher-level requirement you have - why do you think you need the MAC address? As George says, make sure you search before you ask about that.
site:stackoverflow.com your search term hereto find if your question has been asked before. Also, Stack Overflow has a pretty good search, and you can always use that. Also, the "Ask a question" title box pops up a list of questions that may already have your answer. I know both of those questions are in that list.