Sorry for late reply, came across this while searching for other topics
Depends on what distribution you are using.
For linux you have to unpack first then you can run the script files to start the framework and webui.
To unpack type in the command in cli: sudo tar -xvf arachni-****-linux-x86_64.tar.gz
wait until it's done cd into extracted directory usually is arachni and run scripts from bin folder.
For windows it's easier, download exe and save it in the location, run as admin and let it do it's thing, tada from powershell or cmd navigate to arachni directory and run bat files either with normal open from explorer or via cli .\bin\arachni_******