CKAN automatically searches in some default installation paths for KSP, but if you followed my recommendation from the beginning and moved it somewhere else, you need to add it manually. Click on "Add new", navigate to your KSP install and select the "buildID64.txt".
On the first run for a new KSP install, CKAN will also ask you if you want to update the modlist and repository on each launch. I would suggest to allow both options so you are always up-to-date but it's up to you: