It's definitely possible to install Hadoop on a Mac. In fact, almost every developer you see in a Hadoop conference is carrying a Mac :P
To be more specific, Hadoop is targeted for running on Unix and has several modes of operation. In production ("fully distributed mode"), it runs on a cluster of Unix machines, which are usually cheap Linux boxes. In development ("standalone mode"), you run it on a single machine to have quick development cycles. It's very popular to use a Mac when running Hadoop in development mode. I don't know of anyone using Hadoop on a cluster of Mac's though, and I won't be surprised if you find trouble doing that.
Having said all that, just follow the standard instruction in my book or on any Hadoop tutorial to install on the Mac. You'll have to configure your JAVA_HOME property in the Hadoop configuration correctly for your set-up, but otherwise everything should be the same.
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