If you are just now finding out about the beta and don't want to wait until Monday, all you need to do is head over to File>Settings, click "change" under Beta participation, and then choose UI Update. The beta should download automatically and will be available after restarting the client.
Valve is also readying a Mac client of Steam, which will offer native Mac OS X versions of Left 4 Dead, L4D2, Team Fortress 2, Counter-Strike, Portal and the Half-Life series when it launches before the end of April. However, this is expected to come in separate update, so Apple users might still have to wait just a bit longer.
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