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This video will show you how to mine bitcoins on a Mac (The video was made on a Mac running 10.8 Mountain Lion) with a USB Miner (I used a Block Erupter Sapphire 333 MH/s).
There are several steps to this, so follow closely.
First off, make sure you have Xcode installed. It can be downloaded for free from the app store. It is a large download, so it will take a while.
Next, install the command line tools for Xcode. This can be done by opening Xcode, and then clicking the Xcode menu in the toolbar at the top left, and then going to preferences. In the window that pops up, choose the downloads tab, and then double click the command line tools to install them.
After this you will need to install the USB to UART drivers that will allow bfgminer to communicate with the USB miner. That can be downloaded from here: http://www.silabs.com/products/mcu/pages/usbtouartbridgevcpdrivers.aspx
Just make sure to install the Mac OSX driver.
The next step is to download and install Homebrew. Their website here can explain a bit more about what it is: http://brew.sh/.
To install Homebrew, open finder, then click go at the top, and then in the list click utilities, and then in the utilities folder that pops up, choose terminal.
When the terminal opens, copy and past this command in to install homebrew: ruby -e "$(curl -fsSL https://raw.github.com/mxcl/homebrew/go/install)"
After it is finished installing, run the command: brew doctor
This command just shows if there are any problems with the installation and possible fixes. You can just ignore it for now and come back to it if the steps later are having problems.
Next run: brew tap nwoolls/xgminer
This command allows you to install the miner in the next command.
For the next command, run: brew install bfgminer --HEAD
This command will download bfgminer so that you can run it from command line. At this point you are almost done.
Now you can actually run bfgminer. The command for that is: bfgminer -o stratum+tcp://stratum.bitcoin.cz:3333 -u pascal11110.guest -p guest -S all
You just have to put your own information into the command, so you would instead enter: bfgminer -o stratum+tcp://YOUR_MINGING_SITE:3333 -u YOURUSERNAME -p YOURPASSWORD -S all
This will start the miner, but you still have to tell it to use your USB miner. To do this, just hit m, and then hit +. This will direct you to enter search parameters. If everything went correctly, you should just have to enter what it has right above the line your typing in. This should be: erupter:/dev/cu.SLAB_USBtoUART
After this, just hit enter on the USB devices it brings up, and mining should work great.
If you have any issues or I forgot something, just leave a comment below.