According to the SuperCollider documentation, Quarks can be installed as follows:
Unfortunately, that doesn’t actually work.
On OS X, at least, the first time you run this command, it will hang silently while waiting for you to verify the host fingerprint. The very next command you run will return the error. A simple way around this is to open up a terminal and run the following:
svn info https://svn.code.sf.net/p/quarks/code
When prompted, select
(p)ermanently accept. From now on, you will be able to install quarks from within SuperCollider as above. [Tested on SuperCollider 3.6.6]