How do I insert a link that's different for each recipient?

Make sure your CSV file has a "url" column, or something similar. Each recipient should have their own url that goes here. To get around a little trick we'll talk about in a second, make sure these urls don't start with "http://" -- they should have everything after it (ergo "google.com" instead of "http://google.com").

Type out your email and highlight the words where you want the link to appear. Click on the link toolbar icon and enter: "http://{{url}}" -- keeping in mind that if your CSV file used a different column header you'll need to change "url" to match