I found the same thing -- I created a Collection some time ago, but now can't figure out how to add anything else to it. There is an editing button on the page, but it doesn't show anything that would permit new additions.
My guess is that this feature is still more or less "under construction" on eBay.ca, as it just recently showed up on my profile page (I had created the Collection earlier, on .com).
Maybe one of the eBay staffers will pop up and help...
Well, I found the answer to this conundrum, partly by accident, partly by trial and error. It's not quite as intuitive as you might think. You can't apparently add new items once you're already in the Collections view.
What you have to do is find an item you like on eBay -- I assume any current listing - and click on "Add to Collection" near the top of the listing. A screen will pop up that prompts you for the name(s) of your collection(s) so you can choose where you want to save the item (a drop-down box gives you all the Collection names you've set up), then you add a comment if you wish, and click OK. That will put the item into an existing collection.
To start a new collection, first choose an eBay item, go through the same steps, but follow the prompts to create a new collection with a new title.
Once you have one or more Collections, you can edit the comments in them, but evidently you can't add new items from within the feature itself (rather stupid if you ask me, but that appears to be the way it's set up).
By the way, if you leave your Collection public, remember that any comments you add can be viewed by everyone and anyone on eBay.
Here's a screen shot of where to find the "Add to Collection" link, a fairly new addition to the listing page -- it was just below the "Buy It Now" button on my example (one of my own listings 
) --
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