The question is if I am trying to make a file to import using that feature of the software, must I populate all of the fields for it to work or can I just use the key field like the tt number?
You don't have to create the CSV file. The IMDb creates it for you, and Coollector knows how to use it.
For example, here's the page of a user that I took at random:http://www.imdb.com/user/ur20552756/ratings
At the button of the page, there's a link "Export this list". Apparently the IMDb now requires you to be logged in with your IMDb account, otherwise the link isn't shown.