First install brew, a package manager for OSX.
Next install erlang and rebar, the package manager for erlang
brew install erlang
brew install rebar
Use git to download the software, then go into the testnet directory
git clone https://github.com/aeternity/testnet.git
cd testnet
now use rebar to get and compile Aeternity:
rebar get
rebar get-deps
rebar compile
Now you can run your node with sh start.sh