As far as I can see Checkmarks is only accessible via pressing Ctrl + D keys.
The Checkmarks description talks about an additional item "in the drop-down menu of the bookmarks/history sidebar".
I cannot find such an entry in the mentioned sidebar.
So I suggest to let Checkmarks provide a Firefox toolbar icon as almost every other AddOn.
Or at least insert a additional sub menu in Firefox menu "Tools"
This way users would not have to remember on how to launch Checkmarks.