I recently needed to install Siege on my MacBookPro. Here’s what I did:

wget ftp://ftp.joedog.org/pub/siege/siege-latest.tar.gz
tar -xvf siege-latest.tar.gz
cd siege-2.70/
./configure && make && make install

That’s it! Now you should be able to do something like this:

siege -c50 http://yourserver.com

That will simulate 50 concurrent users hitting that particular URL. After siege runs for a bit, you can hit Ctrl+C to kill it and it will output some handy stats about performance.