First build it -- see BUILDING.txt -- then...

-- MacOS --

By convention, ZeroTier One will keep its state here on mac:

/Library/Application Support/ZeroTier/One

ZeroTier ships with a kernel extension for its own tap device, which it
stores in the above directory. To install this from source, type:

sudo make -f Makefile.mac install-mac-tap

This will create the ZeroTier One home above if it does not exist and install
the kext there. Note that the kext must be owned by root:wheel. The make
rule for install-mac-tap takes care of that.

Next, simply run the binary. It must be run as root to open the tap device.
If run with no options, it will use the default home directory above.

sudo ./zerotier-one &

Finally, join the Earth network (the big public LAN and the only net for now):

sudo zerotier-cli join bc8f9a8ee3000001

Try pinging earth.zerotier.net and going to http://earth.zerotier.net/ in a
browser to see if you're online.

-- Linux

On Linux, the default ZeroTier home is:

/var/lib/zerotier-one

Just type:

sudo mkdir /var/lib/zerotier-one
sudo ./zerotier-one &

When run with no options, ZT1 uses its default home path on the current
platform. Note that ZT1 requires the Linux tap driver, so it must be
available in the kernel or as a module. In most Linux distributions it's
included out of the box and should just work.

Finally, join the Earth network (the big public LAN and the only net for now):

sudo zerotier-cli join bc8f9a8ee3000001

Try pinging earth.zerotier.net and going to http://earth.zerotier.net/ in a
browser to see if you're online.

-- Windows

Port in progress, and it's going to pretty much always be more painful to
build than *nix systems. Just wait for the binary release unless you're
brave, in which case you can load the VS2012 solution and play around.